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TABLE OF CONTENTS

As filed with the Securities and Exchange Commission on December 6, 2006

Registration Statement No. 333-133439

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

Post-Effective Amendment No. 1
on Form S-3
to
Registration Statement
on Form S-1
UNDER
THE SECURITIES ACT OF 1933

MarkWest Energy Partners, L.P.
(Exact name of registrant as specified in its charter)

Delaware (State or other jurisdiction of incorporation or organization)	1311 (Primary Standard Industrial Classification Code Number)	27 0005456 (I.R.S. Employer Identification No.)
1515 Arapahoe Street Tower 2, Suite 700 Denver, Colorado 80202 (303) 925-9200 (Address, including zip code, and telephone number, including area code, of registrant's principal executive offices)
Andrew L. Schroeder 1515 Arapahoe Street Tower 2, Suite 700 Denver, Colorado 80202 (303) 925-9200 (Name, address, including zip code, and telephone number, including area code, of agent for service)

Copy to:

David P. Oelman
Vinson & Elkins L.L.P.
2300 First City Tower
1001 Fannin
Houston, Texas 77002-6760
(713) 758-2222

Approximate date of commencement of proposed sale to the public:
From time to time after the effective date of this registration statement.

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box. ý

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. o

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. o

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. o

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until this Registration Statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

THE INFORMATION IN THIS PROSPECTUS IS NOT COMPLETE AND MAY BE CHANGED. WE MAY NOT SELL THESE SECURITIES UNTIL THE REGISTRATION STATEMENT FILED WITH THE SECURITIES AND EXCHANGE COMMISSION IS EFFECTIVE. THIS PROSPECTUS IS NOT AN OFFER TO SELL THESE SECURITIES, AND IT IS NOT SOLICITING AN OFFER TO BUY THESE SECURITIES IN ANY STATE WHERE THE OFFER OR SALE IS NOT PERMITTED.

SUBJECT TO COMPLETION, DATED DECEMBER 6, 2006

PROSPECTUS

3,820,097 Common Units

MarkWest Energy Partners, L.P.

Representing Limited Partner Interests

Up to 3,820,097 of our common units may be offered from time to time by the selling unitholders named in this prospectus. All of the common units were originally sold to the selling unitholders in private placements exempt from the registration requirements of the Securities Act of 1933, as amended. We are registering the offer and sale of the common units to satisfy registration rights that we have granted to the selling unitholders. The selling unitholders may sell the common units at various times and in various types of transactions, including sales in the open market, sales in negotiated transactions and sales by a combination of methods. We are not selling any common units under this prospectus and will not receive any proceeds from the sale of common units by the selling unitholders.

Our common units are traded on the American Stock Exchange under the symbol "MWE." On November 27, 2006, the last reported sale price of our common units on the American Stock Exchange was $53.91 per common unit.

Investing in our common units involves risks. Please read "Risk Factors" beginning on page 2 of this prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus is , 2006

TABLE OF CONTENTS

PROSPECTUS SUMMARY
	MARKWEST ENERGY PARTNERS, L.P.
RISK FACTORS
	Risks Inherent in Our Business
	Risks Related to Our Partnership Structure
	Tax Risks to Common Unitholders
USE OF PROCEEDS
PRICE RANGE OF COMMON UNITS AND DISTRIBUTIONS
UNAUDITED PRO FORMA FINANCIAL STATEMENTS
SELECTED HISTORICAL FINANCIAL INFORMATION
CASH DISTRIBUTION POLICY
	Distributions of Available Cash
	Operating Surplus and Capital Surplus
	Subordination Period
	Distributions of Available Cash from Operating Surplus During the Subordination Period
	Distributions of Available Cash from Operating Surplus After the Subordination Period
	Incentive Distribution Rights
	Target Amount of Quarterly Distribution
	Distributions from Capital Surplus
	Adjustment to the Minimum Quarterly Distribution and Target Distribution Levels
	Distributions of Cash upon Liquidation
SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT
SELLING UNITHOLDERS
CONFLICTS OF INTEREST AND FIDUCIARY DUTIES
THE PARTNERSHIP AGREEMENT
	Organization and Duration
	Purpose
	Power of Attorney
	Capital Contributions
	Limited Liability
	Voting Rights
	Issuance of Additional Securities
	Amendment of the Partnership Agreement
	Action Relating to the Operating Company
	Merger, Sale or Other Disposition of Assets
	Termination and Dissolution
	Liquidation and Distribution of Proceeds
	Withdrawal or Removal of our General Partner
	Transfer of General Partner Interests
	Transfer of Ownership Interests in General Partner
	Transfer of Incentive Distribution Rights
	Change of Management Provisions
	Limited Call Right
	Meetings; Voting
	Status as Limited Partner or Assignee
	Non-citizen Assignees; Redemption
	Indemnification
	Books and Reports
	Right to Inspect our Books and Records
	Registration Rights

MATERIAL TAX CONSEQUENCES
	Partnership Status
	Limited Partner Status
	Tax Consequences of Unit Ownership
	Tax Treatment of Operations
	Disposition of Common Units
	Uniformity of Units
	Tax-Exempt Organizations and Other Investors
	Administrative Matters
	State, Local and Other Tax Considerations
INVESTMENT IN MARKWEST ENERGY PARTNERS BY EMPLOYEE BENEFIT PLANS
DESCRIPTION OF COMMON UNITS
	The Units
	Transfer Agent and Registrar
	Transfer of Common Units
PLAN OF DISTRIBUTION
VALIDITY OF THE COMMON UNITS
EXPERTS
WHERE YOU CAN FIND MORE INFORMATION
FORWARD-LOOKING STATEMENTS
GLOSSARY OF TERMS

You should rely only on the information contained or incorporated by reference in this prospectus. We have not authorized anyone to provide you with information that is different. This prospectus may only be used where it is legal to sell our common units. You should assume that the information contained or incorporated by reference in this prospectus is accurate as of the date on the front of this prospectus only. Our business, financial condition, results of operations and prospects may have changed since that date.

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission (the "SEC") for a continuous offering. Under this prospectus, the selling unitholders may, from time to time, sell the shares of our common units described in this prospectus in one or more offerings. This prospectus may be supplemented from time to time to add, update or change information in this prospectus. Any statement contained in this prospectus will be deemed to be modified or superseded for the purposes of this prospectus to the extent that a statement contained in a prospectus supplement modifies such statement. Any statement so modified will be deemed to constitute a part of this prospectus only as so modified, and any statement so modified will be deemed to constitute a part of this prospectus.

Additional information, including our financial statements for the year ended December 31, 2005 and the nine months ended September 30, 2006 and the notes related thereto, is incorporated by reference to our reports filed with the SEC. In addition, the registration statement containing this prospectus, including the exhibits to the registration statement, provides additional information about us, the selling unitholders and the common units offered under this prospectus. The registration statement, including the exhibits, and our reports filed with the SEC can be read on the SEC website or at the SEC office described under the heading "Where You Can Find More Information."

MarkWest Energy Partners, L.P., MarkWest Hydrocarbon, Inc., our logo and other trademarks mentioned in this prospectus are the property of their respective owners.

PROSPECTUS SUMMARY

This summary highlights information contained elsewhere or incorporated by reference in this prospectus. You should read the entire prospectus and any information incorporated herein by reference carefully, including the historical financial statements and notes to those financial statements incorporated by reference from our Annual Report on Form 10-K and "Unaudited Pro Forma Financial Statements" beginning on page 35 of this prospectus. You should read "Risk Factors" beginning on page 2 for more information about important factors that you should consider before buying common units. We include a glossary of some of the terms used in this prospectus as Appendix A to this prospectus.

Unless the context otherwise requires, references in this prospectus to "MarkWest Energy," "the Partnership," "we," "us," "our" or "ours" refer to MarkWest Energy Partners, L.P., together with its consolidated subsidiaries. When the context requires, we refer to these entities separately. References in this prospectus to "MarkWest Hydrocarbon" refer to MarkWest Hydrocarbon, Inc. and its direct and indirect consolidated subsidiaries.

MARKWEST ENERGY PARTNERS, L.P.

Overview

We are a publicly traded Delaware limited partnership engaged in the gathering, transportation and processing of natural gas; the transportation, fractionation and storage of NGLs; and the gathering and transportation of crude oil. We are the largest processor of natural gas in the Appalachia region. We also have a large natural gas gathering and transmission business in the southwestern United States, built primarily through acquisitions and investments: Pinnacle Natural Gas, the Lubbock transmission pipeline and the Foss Lake gathering system, all in 2003; the Carthage gathering system in East Texas in July 2004; and in 2005, the Javelina Processing Facilities in Corpus Christi, Texas, and a non-controlling 50% interest in Starfish Pipeline Company, LLC in southern Louisiana and the Gulf of Mexico.

We generate revenues by providing gathering, processing, transportation, fractionation, and storage services. We believe that the largely fee-based nature of our business and the relatively long-term nature of our contracts provide a relatively stable base of cash flows. As a publicly traded partnership, we have access to, and regularly utilize, both equity and debt capital markets as a source of financing, as well as that provided by our credit facility. Our limited partnership structure also provides tax advantages to our unitholders.

Our principal executive offices are located at 1515 Arapahoe St., Tower II, Suite 700, Denver, Colorado 80202 in July 2006 and our phone number is (303) 925-9200. We maintain a website at http://www.markwest.com. Except for information specifically incorporated by reference into this prospectus that may be accessed from our website, the information on our website is not part of this prospectus, and you should rely only on information contained or incorporated by reference in this prospectus when making a decision as to whether or not to invest in our common units.

RISK FACTORS

You should carefully consider each of the risks described below, together with all of the other information contained or incorporated by reference in this prospectus, before deciding to invest in shares of our common stock. If any of the following risks develop into actual events, our business, financial condition or results of operations could be materially adversely affected and you may lose all or part of your investment.

Risks Inherent in Our Business

We may not have sufficient cash after the establishment of cash reserves and payment of our general partner's fees and expenses to enable us to pay distributions at the current level.

We may not have sufficient available cash from operating surplus each quarter to pay the minimum quarterly distribution. The amount of cash we can distribute on our units depends principally on the amount of cash we generate from our operations, which will fluctuate from quarter to quarter based on, among other things:

•: the fees we charge and the margins we realize for our services and sales;
•: the prices of, level of production of, and demand for natural gas and NGLs;
•: the volumes of natural gas we gather, process and transport;
•: the level of our operating costs, including reimbursement of fees and expenses of our general partner; and
•: prevailing economic conditions.

In addition, the actual amount of cash we will have available for distribution will depend on other factors, some of which are beyond our control, including:

•: our debt service requirements;
•: fluctuations in our working capital needs;
•: our ability to borrow funds and access capital markets;
•: restrictions contained in our debt agreements;
•: the level of capital expenditures we make, including capital expenditures incurred in connection with our enhancement projects;
•: the cost of acquisitions, if any; and
•: the amount of cash reserves established by our general partner.

You should be aware that the amount of cash we have available for distribution depends primarily on our cash flow and not solely on profitability, which is affected by non-cash items. As a result, we may make cash distributions during periods when we record losses for financial accounting purposes and may not make cash distributions during periods when we record net earnings for financial accounting purposes.

If we do not make acquisitions on economically acceptable terms, our future growth will be limited.

Our ability to grow depends in part on our ability to make acquisitions that result in an increase in the cash generated from operations per unit. If we are unable to make these accretive acquisitions because we are (1) unable to identify attractive acquisition candidates or negotiate acceptable purchase contracts with them, (2) unable to obtain financing for these acquisitions on economically acceptable

terms, or (3) outbid by competitors, then our future growth and ability to increase distributions will be limited.

If we are unable to successfully integrate our recent or future acquisitions, our future financial performance may suffer.

Our future growth will depend in part on our ability to integrate our recent acquisitions. We cannot guarantee that we will successfully integrate these, or any other, acquisitions into our existing operations, or that we will achieve the desired profitability and anticipated results from such acquisitions. Failure to achieve such planned results could adversely affect our financial condition and results of operations.

The integration of acquisitions with our existing business involves numerous risks, including:

•: operating a significantly larger combined organization and integrating additional midstream operations into our existing operations;
•: difficulties in the assimilation of the assets and operations of the acquired businesses, especially if the assets acquired are in a new business segment or geographical area;
•: the loss of customers or key employees from the acquired businesses;
•: the diversion of management's attention from other existing business concerns;
•: the failure to realize expected synergies and cost savings;
•: coordinating geographically disparate organizations, systems and facilities;
•: integrating personnel from diverse business backgrounds and organizational cultures; and
•: consolidating corporate and administrative functions.

Further, unexpected costs and challenges may arise whenever businesses with different operations or management are combined, and we may experience unanticipated delays in realizing the benefits of an acquisition. Following an acquisition, we may discover previously unknown liabilities including those under the same stringent environmental laws and regulations relating to releases of pollutants into the environment and environmental protection as applicable to our existing plants, pipelines and facilities. If so, our operation of these new assets could cause us to incur increased costs to address these liabilities or attain or maintain compliance with such requirements. If we consummate any future acquisition, our capitalization and results of operation may change significantly, and unitholders will not have the opportunity to evaluate the economic, financial and other relevant information that we will consider in determining the application of these funds and other resources.

Our acquisition strategy is based in part on our expectation of ongoing divestitures of assets within the midstream petroleum and natural gas industry. A material decrease in such divestitures could limit our opportunities for future acquisitions, and could adversely affect our operations and cash flows available for distribution to our unitholders.

Growing our business by constructing new pipelines and processing and treating facilities subjects us to construction risks and risks that natural gas supplies will not be available upon completion of the facilities.

One of the ways we intend to grow our business is through the construction of additions to our existing gathering systems and construction of new gathering, processing and treating facilities. The construction of gathering, processing and treating facilities requires the expenditure of significant amounts of capital, which may exceed our expectations, and involves numerous regulatory, environmental, political and legal uncertainties. If we undertake these projects, we may not be able to

complete them on schedule or at all or at the budgeted cost. Moreover, our revenues may not increase immediately upon the expenditure of funds on a particular project. For instance, if we build a new pipeline, the construction will occur over an extended period of time, and we will not receive any material increases in revenues until after completion of the project.

Furthermore, we may have only limited natural gas supplies committed to these facilities prior to their construction. Moreover, we may construct facilities to capture anticipated future growth in production in a region in which anticipated production growth does not materialize. We may also rely on estimates of proved reserves in our decision to construct new pipelines and facilities, which may prove to be inaccurate because there are numerous uncertainties inherent in estimating quantities of proved reserves. As a result, new facilities may not be able to attract enough natural gas to achieve our expected investment return, which could adversely affect our results of operations and financial condition.

Our substantial debt and other financial obligations could impair our financial condition, results of operations and cash flows, and our ability to fulfill our debt obligations.

We have substantial indebtedness and other financial obligations. Subject to the restrictions governing our indebtedness and other financial obligations, and the indenture governing our outstanding notes, we may incur significant additional indebtedness and other financial obligations.

Our substantial indebtedness and other financial obligations could have important consequences. For example, they could:

•: make it more difficult for us to satisfy our obligations with respect to our existing debt;
•: impair our ability to obtain additional financings in the future for working capital, capital expenditures, acquisitions, or general partnership and other purposes;
•: have a material adverse effect on us if we fail to comply with financial and restrictive covenants in our debt agreements, and an event of default occurs as a result of that failure that is not cured or waived;
•: require us to dedicate a substantial portion of our cash flow to payments on our indebtedness and other financial obligations, thereby reducing the availability of our cash flow to fund working capital, capital expenditures, distributions and other general partnership requirements;
•: limit our flexibility in planning for, or reacting to, changes in our business and the industry in which we operate; and
•: place us at a competitive disadvantage compared to our competitors that have proportionately less debt.

Furthermore, these consequences could limit our ability, and the ability of our subsidiaries, to obtain future financings, make needed capital expenditures, withstand a future downturn in our business or the economy in general, conduct operations or otherwise take advantage of business opportunities that may arise. Our existing credit facility contains covenants requiring us to maintain specified financial ratios and satisfy other financial conditions, which may limit our ability to grant liens on our assets, make or own certain investments, enter into any swap contracts other than in the ordinary course of business, merge, consolidate or sell assets, incur indebtedness senior to our credit facility, make distributions on equity investments, and declare or make, directly or indirectly, any distribution on our common units. Our obligations under the credit facility are secured by substantially all of our assets and guaranteed by us and all of our subsidiaries, other than our operating company, which is the borrower under the credit facility. Please read "Item 7. Management's Discussion and Analysis of Financial Condition and Results of Operations—Liquidity and Capital Resources" in our most recent Annual Report on Form 10-K, as amended, incorporated by reference herein. We may be

unable to meet those ratios and conditions. Any future breach of any of these covenants or our failure to meet any of these ratios or conditions could result in a default under the terms of our credit facility, which could result in acceleration of our debt and other financial obligations. If we were unable to repay those amounts, the lenders could initiate a bankruptcy or liquidation proceeding, or proceed against the collateral.

A significant decrease in natural gas production in our areas of operation would reduce our ability to make distributions to our unitholders.

Our gathering systems are connected to natural gas reserves and wells, from which the production will naturally decline over time, which means that our cash flows associated with these wells will also decline over time. To maintain or increase throughput levels on our gathering systems and the utilization rate at our processing plants and our treating and fractionation facilities, we must continually obtain new natural gas supplies. Our ability to obtain additional sources of natural gas depends in part on the level of successful drilling activity near our gathering systems.

We have no control over the level of drilling activity in the areas of our operations, the amount of reserves associated with the wells or the rate at which production from a well will decline. In addition, we have no control over producers or their production decisions, which are affected by, among other things, prevailing and projected energy prices, demand for hydrocarbons, the level of reserves, geological considerations, governmental regulations and the availability and cost of capital. Fluctuations in energy prices can greatly affect production rates and investments by third parties in the development of new oil and natural gas reserves. Drilling activity generally decreases as oil and natural gas prices decrease.

Because of these factors, even if new natural gas reserves are discovered in areas served by our assets, producers may choose not to develop those reserves. If we are not able to obtain new supplies of natural gas to replace the natural decline in volumes from existing wells due to reductions in drilling activity or competition, throughput on our pipelines and the utilization rates of our treating and processing facilities would decline, which could have a material adverse effect on our business, results of operations, financial condition and ability to make cash distributions to you.

We depend on third parties for the natural gas and refinery off-gas we process, and the NGLs we fractionate at our facilities, and a reduction in these quantities could reduce our revenues and cash flow.

Although we obtain our supply of natural gas, refinery off-gas and NGLs from numerous third-party producers, a significant portion comes from a limited number of key producers/suppliers who are committed to us under processing contracts. According to these contracts or other supply arrangements, however, the producers are under no obligation to deliver a specific quantity of natural gas or NGLs to our facilities. If these key suppliers, or a significant number of other producers, were to decrease the supply of natural gas or NGLs to our systems and facilities for any reason, we could experience difficulty in replacing those lost volumes. Because our operating costs are primarily fixed, a reduction in the volumes of natural gas or NGLs delivered to us would result not only in a reduction of revenues, but also a decline in net income and cash flow of similar magnitude.

We derive a significant portion of our revenues from our gas processing, transportation, fractionation and storage agreements with MarkWest Hydrocarbon, and its failure to satisfy its payment or other obligations under these agreements could reduce our revenues and cash flow.

MarkWest Hydrocarbon accounts for a significant portion of our revenues and net operating margin. These revenues and margins are generated by the volumes of natural gas contractually committed to MarkWest Hydrocarbon by certain producers in the Appalachian region, as well as the

fees generated from processing, transportation, fractionation and storage services provided to MarkWest Hydrocarbon. We expect to derive a significant portion of our revenues and net operating margin from the services we provide under our contracts with MarkWest Hydrocarbon for the foreseeable future. Any default or nonperformance by MarkWest Hydrocarbon could significantly reduce our revenues and cash flows. Thus, any factor or event adversely affecting MarkWest Hydrocarbon's business, creditworthiness or its ability to perform under its contracts with us, or its other contracts related to our business, could also adversely affect us.

The fees charged to third parties under our gathering, processing, transmission, transportation, fractionation and storage agreements may not escalate sufficiently to cover increases in costs. The agreements may not be renewed or may be suspended in some circumstances.

Our costs may increase at a rate greater than the fees we charge to third parties. Furthermore, third parties may not renew their contracts with us. Additionally, some third parties' obligations under their agreements with us may be permanently or temporarily reduced due to certain events, some of which are beyond our control, including force majeure events wherein the supply of either natural gas, NGLs or crude oil are curtailed or cut off. Force majeure events include (but are not limited to): revolutions, wars, acts of enemies, embargoes, import or export restrictions, strikes, lockouts, fires, storms, floods, acts of God, explosions, mechanical or physical failures of equipment or our facilities or facilities of third parties. If the escalation of fees is insufficient to cover increased costs, if third parties do not renew or extend their contracts with us or if any third party suspends or terminates its contracts with us, our financial results would suffer.

We are exposed to the credit risks of our key customers, and any material nonpayment or nonperformance by our key customers could reduce our ability to make distributions to our unitholders.

We are subject to risks of loss resulting from nonpayment or nonperformance by our customers. Any material nonpayment or nonperformance by our key customers could reduce our ability to make distributions to our unitholders. Furthermore, some of our customers may be highly leveraged and subject to their own operating and regulatory risks, which increases the risk that they may default on their obligations to us.

We may not be able to retain existing customers, or acquire new customers, which would reduce our revenues and limit our future profitability.

The renewal or replacement of existing contracts with our customers at rates sufficient to maintain current revenues and cash flows depends on a number of factors beyond our control, including competition from other gatherers, processors, pipelines, fractionators, and the price of, and demand for, natural gas, NGLs and crude oil in the markets we serve. Our competitors include large oil, natural gas, refining and petrochemical companies, some of which have greater financial resources, more numerous or greater capacity pipelines, processing and other facilities, and greater access to natural gas and NGL supplies than we do. Additionally, our customers that gather gas through facilities that are not otherwise dedicated to us may develop their own processing and fractionation facilities in lieu of using our services. Certain of our competitors may also have advantages in competing for acquisitions, or other new business opportunities, because of their financial resources and synergies in operations.

As a consequence of the increase in competition in the industry, and the volatility of natural gas prices, end-users and utilities are reluctant to enter into long-term purchase contracts. Many end-users purchase natural gas from more than one natural gas company and have the ability to change providers at any time. Some of these end-users also have the ability to switch between gas and alternative fuels in response to relative price fluctuations in the market. Because there are numerous companies of greatly varying size and financial capacity that compete with us in the marketing of natural gas, we often compete in the end-user and utilities markets primarily on the basis of price. The inability of our

management to renew or replace our current contracts as they expire and to respond appropriately to changing market conditions could affect our profitability. For more information regarding our competition, please read "Business—Industry Overview" as set forth in our most recent Annual Report on Form 10-K incorporated by reference herein.

Our profitability is affected by the volatility of NGL product and natural gas prices.

We are subject to significant risks associated with frequent and often substantial fluctuations in commodity prices. In the past, the prices of natural gas and NGLs have been extremely volatile, and we expect this volatility to continue. The NYMEX daily settlement price of natural gas for the prompt month contract in 2005 ranged from a high of $15.38 per MMBtu to a low of $5.79 per MMBtu in 2005. In the first nine months of 2006, the same index ranged from a high of $11.23 per MMBtu to a low of $4.20 per MMBtu. A composite of the weighted monthly average NGLs price at our Appalachian facilities based on our average NGLs composition in 2005 ranged from a high of approximately $1.25 per gallon to a low of $0.83 per gallon. In the first nine months of 2006, the same composite ranged from approximately $1.31 per gallon to approximately $1.02 per gallon. The markets and prices for natural gas and NGLs depend upon factors beyond our control. These factors include demand for oil, natural gas and NGLs, which fluctuate with changes in market and economic conditions and other factors, including:

•: the level of domestic oil, natural gas and NGL production;
•: demand for natural gas and NGL products in localized markets;
•: imports of crude oil, natural gas and NGLs;
•: seasonality;
•: the condition of the U.S. economy;
•: political conditions in other oil-producing and natural gas-producing countries; and
•: domestic government regulation, legislation and policies.

Our net operating margins under various types of commodity-based contracts are directly affected by changes in NGL product prices and natural gas prices, thus are more sensitive to volatility in commodity prices than our fee-based contracts. Additionally, our purchase and resale of gas in the ordinary course of business exposes us to significant risk of volatility in gas prices due to the potential difference in the time of the purchases and sales, and the existence of a potential difference in the gas price associated with each transaction.

Our commodity derivative activities may reduce our earnings, profitability and cash flows.

Our operations expose us to fluctuations in commodity prices. We utilize derivative financial instruments related to the future price of crude oil, natural gas and certain NGLs with the intent of reducing volatility in our cash flows due to fluctuations in commodity prices.

We account for derivative instruments in accordance with the provisions of Statement of Financial Accounting Standards (SFAS) No. 133, Accounting for Derivative Instruments and Hedging Activities. The extent of our commodity price exposure is related largely to the effectiveness and scope of our derivative activities. We have a policy to enter into derivative transactions related to only a portion of the volume of our expected production or fuel requirements and, as a result, we will continue to have direct commodity price exposure to the unhedged portion. Our actual future production or fuel requirements may be significantly higher or lower than we estimate at the time we enter into derivative transactions for such period. If the actual amount is higher than we estimate, we will have greater commodity price exposure than we intended. If the actual amount is lower than the amount that is

subject to our derivative financial instruments, we might be forced to satisfy all or a portion of our derivative transactions without the benefit of the cash flow from our sale or purchase of the underlying physical commodity, resulting in a substantial diminution of our liquidity. As a result of these factors, our hedging activities may not be as effective as we intend in reducing the volatility of our cash flows, and in certain circumstances may actually increase the volatility of our cash flows. In addition, our hedging activities are subject to the risks that a counterparty may not perform its obligation under the applicable derivative instrument, the terms of the derivative instruments are imperfect, and our hedging policies and procedures are not properly followed. It is possible that the steps we take to monitor our derivative financial instruments may not detect and prevent violations of our risk management policies and procedures, particularly if deception or other intentional misconduct is involved. For further information about our risk management policies and procedures, please read "Item 7. Management's Discussion and Analysis of Financial Condition and Results of Operation — Quantitative and Qualitative Disclosures About Market Risk — Commodity Price Risk and Our Risk Management Policy" as set forth in our most recent Annual Report on Form 10-K, as amended, incorporated by reference herein.

We have found material weaknesses in our internal controls that require remediation and concluded, pursuant to Section 404 of the Sarbanes-Oxley Act of 2002, that our internal controls over financial reporting at December 31, 2005, 2004 and 2003, were not effective. If we fail to remediate any material weaknesses, we could be unable to provide timely and reliable financial information, which could have a material adverse effect on our business, results of operations or financial condition.

We have identified, and the audit report on management's assessment of the effectiveness of internal control over financial reporting and the effectiveness of internal control over financial reporting of Deloitte & Touche LLP as of December 31, 2005 confirmed the presence of, material weaknesses in our internal controls over financial reporting. In particular, our control environment did not sufficiently promote effective internal control over financial reporting through the management structure to prevent a material misstatement. Furthermore, we did not have adequate internal controls and processes in place to allow independent validation of data or control and review of our management's assertions with respect to the completeness, accuracy and validity of commodity derivative transactions.

In addition, we and KPMG LLP, our independent registered public accounting firm at that time, identified material weaknesses in our internal control over financial reporting as of December 31, 2004. Additionally, PwC, our independent registered public accounting firm at the time, identified certain deficiencies in our internal accounting controls as of December 31, 2003. Considered collectively, these deficiencies may have constituted a material weakness in our internal controls pursuant to standards established by the American Institute of Certified Public Accountants. For a further discussion of these material weaknesses, please read "Item 7. Managements Discussion and Analysis of Financial Condition and Results of Operations — Overview — Material Weaknesses Reported for the Years Ended December 31, 2005, 2004 and 2003" as set forth in our most recent Annual Report on Form 10-K incorporated by reference herein and "Item 4. Controls and Procedures" as set forth in our most recent Quarterly Report on Form 10-Q incorporated by reference herein.

The full impact of our efforts to remediate the identified material weaknesses had not been realized as of December 31, 2005 and may not be sufficient to maintain effective internal controls in the future. We may not be able to implement and maintain adequate controls over our financial processes and reporting, which may require us to restate our financial statements in the future. In addition, we may discover additional past, ongoing or future material weaknesses or significant deficiencies in our financial reporting system in the future. Any failure to implement new controls, or difficulty encountered in their implementation, could cause us to fail to meet our reporting obligations or result in material misstatements in our financial statements. Inferior internal controls could also

cause investors to lose confidence in our reported financial information, which could result in a lower trading price of our common units.

We are subject to operating and litigation risks that may not be covered by insurance.

Our industry is subject to numerous operating hazards and risks incidental to processing, transporting, fractionating and storing natural gas and NGLs, and to transporting and storing crude oil. These include:

•: damage to pipelines, plants, related equipment and surrounding properties caused by floods, hurricanes and other natural disasters;
•: inadvertent damage from construction and farm equipment;
•: leakage of crude oil, natural gas, NGLs and other hydrocarbons;
•: fires and explosions; and
•: other hazards, including those associated with high-sulfur content, or sour gas, that could also result in personal injury and loss of life, pollution and suspension of operations.

As a result, we may be a defendant in various legal proceedings and litigation arising from our operations. We may not be able to maintain or obtain insurance of the type and amount we desire at reasonable rates. Market conditions could cause certain insurance premiums and deductibles to become unavailable, or available only for reduced amounts of coverage. For example, insurance carriers now require broad exclusions for losses due to war risk and terrorist acts. In addition, insurance carriers have indicated hurricane insurance premiums may increase significantly and policy limits may be significantly reduced. If we were to incur a significant liability for which we were not fully insured, it could have a material adverse effect on our financial position.

Transportation on certain of our pipelines may be subject to federal or state rate and service regulation, and the imposition and/or cost of compliance with such regulation could adversely affect our profitability.

Some of our gas, liquids and crude oil transmission operations are subject to rate and service regulations under FERC or various state regulatory bodies, depending upon jurisdiction. FERC generally regulates the transportation of natural gas and oil in interstate commerce, and FERC's regulatory authority includes: facilities construction, acquisition, extension or abandonment of services or facilities; accounts and records; and depreciation and amortization policies. Intrastate natural gas pipeline operations and transportation on proprietary natural gas or petroleum products pipelines are generally not subject to regulation by FERC, and the Natural Gas Act ("NGA") specifically exempts some gathering systems. Yet such operations may still be subject to regulation by various state agencies. The applicable statutes and regulations generally require that our rates and terms and conditions of service provide no more than a fair return on the aggregate value of the facilities used to render services. We cannot assure you that FERC will not at some point determine that such gathering and transportation services are within its jurisdiction, and regulate such services. FERC rate cases can involve complex and expensive proceedings. For more information regarding regulatory matters that could affect our business, please read "Item 1. Business — Regulatory Matters" as set forth in our most recent Annual Report on Form 10-K incorporated by reference herein.

If we are unable to obtain new rights-of-way or the cost of renewing existing rights-of-way increases, then we may be unable to fully execute our growth strategy and our cash flows could be adversely affected.

The construction of additions to our existing gathering assets may require us to obtain new rights-of-way prior to constructing new pipelines. We may be unable to obtain such rights-of-way to connect new natural gas supplies to our existing gathering lines or capitalize on other attractive expansion opportunities. Additionally, it may become more expensive for us to obtain new rights-of-way or to renew existing rights-of-way. If the cost of obtaining new rights-of-way or renewing existing rights-of-way increases, then our cash flows could be adversely affected.

We are indemnified for liabilities arising from an ongoing remediation of property on which our facilities are located and our results of operation and our ability to make payments of principal and interest on the notes could be adversely affected if the indemnifying party fails to perform its indemnification obligation.

Columbia Gas is the previous or current owner of the property on which our Kenova, Boldman, Cobb and Kermit facilities are located and is the previous operator of our Boldman and Cobb facilities. Columbia Gas has been or is currently involved in investigatory or remedial activities with respect to the real property underlying the Boldman and Cobb facilities pursuant to an "Administrative Order by Consent for Removal Actions" entered into by Columbia Gas and the U.S. Environmental Protection Agency and, in the case of the Boldman facility, an "Agreed Order" with the Kentucky Natural Resources and Environmental Protection Cabinet.

Columbia Gas has agreed to retain sole liability and responsibility for, and to indemnify MarkWest Hydrocarbon against, any environmental liabilities associated with these regulatory orders or the real property underlying these facilities to the extent such liabilities arose prior to the effective date of the agreements pursuant to which such properties were acquired or leased from Columbia Gas. At the closing of our initial public offering, MarkWest Hydrocarbon assigned us the benefit of its indemnity from Columbia Gas with respect to the Cobb, Boldman and Kermit facilities. While we are not a party to the agreement under which Columbia Gas agreed to indemnify MarkWest Hydrocarbon with respect to the Kenova facility, MarkWest Hydrocarbon has agreed to provide to us the benefit of its indemnity, as well as any other third party environmental indemnity of which it is a beneficiary. MarkWest Hydrocarbon has also agreed to provide us an additional environmental indemnity pursuant to the terms of the Omnibus Agreement. Our results of operation and our ability to make cash distributions to our unitholders could be adversely affected if in the future either Columbia Gas or MarkWest Hydrocarbon fails to perform under the indemnification provisions of which we are the beneficiary.

Our business is subject to federal, state and local laws and regulations with respect to environmental, safety and other regulatory matters, and the violation of, or the cost of compliance with, such laws and regulations could adversely affect our profitability.

Numerous governmental agencies enforce comprehensive and stringent laws and regulations on a wide range of environmental, safety and other regulatory matters. We could be adversely affected by increased costs due to stricter pollution-control requirements or liabilities resulting from non-compliance with operating or other regulatory permits. New environmental laws and regulations might adversely influence our products and activities. Federal, state and local agencies also could impose additional safety requirements, any of which could affect our profitability. In addition, we face the risk of accidental releases or spills associated with our operations. These could result in material costs and liabilities, including those relating to claims for damages to property and persons. Our failure to comply with environmental or safety-related laws and regulations could result in administrative, civil and criminal penalties, the imposition of investigatory and remedial obligations and even injunctions that restrict or prohibit our operations. For more information regarding the environmental, safety and

other regulatory matters that could affect our business, please read "Business—Regulatory Matters," "Business—Environmental Matters" and "Business—Pipeline Safety Regulations" each as set forth in our most recent Annual Report on Form 10-K incorporated by reference herein.

The amount of gas we process, gather and transmit, or the crude oil we gather and transport, may be reduced if the pipelines to which we deliver the natural gas or crude oil cannot, or will not, accept the gas or crude oil.

All of the natural gas we process, gather and transmit is delivered into pipelines for further delivery to end-users. If these pipelines cannot, or will not, accept delivery of the gas due to downstream constraints on the pipeline, we will be forced to limit or stop the flow of gas through our pipelines and processing systems. In addition, interruption of pipeline service upstream of our processing facilities would likewise limit or stop flow through our processing facilities. Likewise, if the pipelines into which we deliver crude oil are interrupted, we will be limited in, or prevented from conducting, our crude oil transportation operations. Any number of factors beyond our control could cause such interruptions or constraints on pipeline service, including necessary and scheduled maintenance, or unexpected damage to the pipeline. Because our revenues and net operating margins depend upon (1) the volumes of natural gas we process, gather and transmit, (2) the throughput of NGLs through our transportation, fractionation and storage facilities and (3) the volume of crude oil we gather and transport, any reduction of volumes could result in a material reduction in our net operating margin.

Our business would be adversely affected if operations at any of our facilities were interrupted.

Our operations depend upon the infrastructure that we have developed, including processing and fractionation plants, storage facilities, and various means of transportation. Any significant interruption at these facilities or pipelines, or our inability to transmit natural gas or NGLs, or transport crude oil to or from these facilities or pipelines for any reason, would adversely affect our results of operations. Operations at our facilities could be partially or completely shut down, temporarily or permanently, as the result of circumstances not within our control, such as:

•: unscheduled turnarounds or catastrophic events at our physical plants;
•: labor difficulties that result in a work stoppage or slowdown; and
•: a disruption in the supply of crude oil to our crude oil pipeline, natural gas to our processing plants or gathering pipelines, or a disruption in the supply of NGLs to our transportation pipeline and fractionation facility.

Due to our lack of asset diversification, adverse developments in our gathering, processing, transportation, transmission, fractionation and storage businesses would reduce our ability to make distributions to our unitholders.

We rely exclusively on the revenues generated from our gathering, processing, transportation, transmission, fractionation and storage businesses. An adverse development in one of these businesses would have a significantly greater impact on our financial condition and results of operations than if we maintained more diverse assets.

As a result of damage caused by Hurricanes Katrina and Rita in the Gulf of Mexico and Gulf Coast regions, insurance costs related to oil and gas assets in these regions have increased significantly. We may be unable to obtain insurance on our interest in Starfish at rates we consider reasonable.

During 2005, Hurricanes Katrina and Rita caused severe and widespread damage to oil and gas assets in the Gulf of Mexico and Gulf Coast regions. The resulting loss to both offshore and onshore

assets led to substantial insurance claims within the oil and gas industry. Along with other industry participants, we expect insurance costs to increase within this region as a result of these developments. We have renewed our insurance coverage relating to Starfish and mitigated a portion of the cost increase by reducing coverage and broadening the self-insurance element of our overall coverage. We may be unable to obtain insurance on our interest in Starfish at rates we consider reasonable and as a result may experience losses that are not insured or that exceed the maximum limits under our insurance policies. If a significant negative event that is not fully insured occurs with respect to Starfish, it could materially and adversely affect our financial condition and results of operations.

A shortage of skilled labor may make it difficult for us to maintain labor productivity, and competitive costs and could adversely affect our profitability.

Our operations require skilled and experienced laborers with proficiency in multiple tasks. In recent years, a shortage of workers trained in various skills associated with the midstream energy business has caused us to conduct certain operations without full staff, which decreases our productivity and increases our costs. This shortage of trained workers is the result of the previous generation's experienced workers reaching the age for retirement, combined with the difficulty of attracting new laborers to the midstream energy industry. Thus, this shortage of skilled labor could continue over an extended period. If the shortage of experienced labor continues or worsens, it could have an adverse impact on our labor productivity and costs and our ability to expand production in the event there is an increase in the demand for our products and services, which could adversely affect our profitability.

Risks Related to Our Partnership Structure

Cost reimbursements and fees due our general partner may be substantial and reduce our cash available for distribution to unitholders.

Prior to making any distribution on the common units, we reimburse our general partner for all expenses it incurs on our behalf. Our general partner has sole discretion in determining the amount of these expenses. Our general partner and its affiliates also may provide us other services for which we will be charged fees.

MarkWest Hydrocarbon and its affiliates have conflicts of interest and limited fiduciary duties, which may permit them to favor their own interests to the detriment of the unitholders.

MarkWest Hydrocarbon and its affiliates own and control our general partner. MarkWest Hydrocarbon and its affiliates also own a significant limited partner interest in us. A number of officers and employees of MarkWest Hydrocarbon and our general partner also own interests in us. Conflicts of interest may arise between MarkWest Hydrocarbon and its affiliates, including us and our general partner. As a result of these conflicts, our general partner may favor its own interests and the interests of its affiliates including our general partner on the one hand, and us and our unitholders, on the other hand. These conflicts include, among others, the following situations:

•: Employees of MarkWest Hydrocarbon who provide services to us also devote significant time to the businesses of MarkWest Hydrocarbon and are compensated by MarkWest Hydrocarbon for these services.
•: Neither our Partnership Agreement nor any other agreement requires MarkWest Hydrocarbon to pursue a future business strategy that favors us or utilizes our assets for processing, transportation or fractionation services we provide. MarkWest Hydrocarbon's directors and officers have a fiduciary duty to make these decisions in the best interests of the stockholders of MarkWest Hydrocarbon.

•: Our general partner is allowed to take into account the interests of other parties, such as MarkWest Hydrocarbon, in resolving conflicts of interest, which has the effect of limiting its fiduciary duty to our unitholders.
•: Our general partner may limit its liability and reduce its fiduciary duties, while also restricting the remedies available to our unitholders for actions that, without the limitations, might constitute breaches of fiduciary duty. As a result of purchasing units, our unitholders consent to some actions and conflicts of interest that might otherwise constitute a breach of fiduciary or other duties under applicable state law.
•: Our general partner controls the enforcement of obligations owed to us by our general partner and its affiliates, including the processing, transportation and fractionation agreements with MarkWest Hydrocarbon.
•: Our general partner decides whether to retain separate counsel, accountants or others to perform services for us
•: In some instances, our general partner may cause us to borrow funds in order to make cash distributions, even if the purpose or effect of the borrowing is to make a distribution on the subordinated units or to make incentive distributions or to hasten the conversion of subordinated units.
•: Our Partnership Agreement gives our general partner broad discretion in establishing financial reserves for the proper conduct of our business. These reserves also will affect the amount of cash available for distribution. Our general partner may establish reserves for distribution on the subordinated units, but only if those reserves will not prevent us from distributing the full minimum quarterly distribution, plus any arrearages, on the common units for the following four quarters.
•: Our general partner determines the amount and timing of asset purchases and sales, capital expenditures, borrowings, issuance of additional partnership securities and reserves, each of which can affect the amount of cash that is distributed to our unitholders.
•: Our general partner determines which costs incurred by MarkWest Hydrocarbon and its affiliates are reimbursable by us.
•: Our Partnership Agreement does not restrict our general partner from causing us to pay it or its affiliates for any services rendered on terms that are fair and reasonable to us or entering into additional contractual arrangements with any of these entities on our behalf.

MarkWest Hydrocarbon and its affiliates are not limited in their ability to compete with us, which could cause conflicts of interest and limit our ability to acquire additional assets or businesses and which in turn could adversely affect our results of operations and cash available for distribution to our unitholders.

Neither our partnership agreement nor the omnibus agreement among us, MarkWest Hydrocarbon and others prohibits MarkWest Hydrocarbon and its affiliates from owning assets or engaging in businesses that compete directly or indirectly with us. In addition, MarkWest Hydrocarbon and its affiliates may acquire, construct or dispose of additional midstream or other assets in the future, without any obligation to offer us the opportunity to purchase or construct any of those assets. MarkWest Hydrocarbon is a large, established participant in the midstream energy business and has significantly greater resources and experience than we have, which may make it more difficult for us to compete with it with respect to commercial activities as well as for acquisition candidates. As a result, competition from these entities could adversely impact our results of operations and cash available for distribution. Please read "Conflicts of Interest and Fiduciary Duties."

Unitholders have less ability to elect or remove management than holders of common stock in a corporation.

Unlike the holders of common stock in a corporation, unitholders have only limited voting rights on matters affecting our business, and therefore limited ability to influence management's decisions regarding our business. Unitholders did not elect our general partner or its board of directors, and have no right to elect our general partner or its board of directors on an annual or other continuing basis.

MarkWest Hydrocarbon and its affiliates choose the board of directors of our general partner. The directors of our general partner also have a fiduciary duty to manage our general partner in a manner beneficial to its members, MarkWest Hydrocarbon and its affiliates.

Furthermore, if unitholders are dissatisfied with the performance of our general partner, they have little ability to remove our general partner. First, our general partner generally may not be removed except upon the vote of the holders of at least 66²/3% of the outstanding units voting together as a single class. Also, if our general partner is removed without cause during the subordination period, and units held by MarkWest Hydrocarbon and its affiliates are not voted in favor of that removal, all remaining subordinated units will automatically be converted into common units, and any existing arrearages on the common units will be extinguished. A removal under these circumstances would adversely affect the common unitholders by prematurely eliminating their contractual right to distributions over the subordinated units, which would otherwise have continued until we had met certain distribution and performance tests.

Cause is narrowly defined to mean that a court of competent jurisdiction has entered a final, non-appealable judgment finding our general partner liable for actual fraud, gross negligence, or willful or wanton misconduct in its capacity as our general partner. Cause does not include most cases of charges of poor management of the business, so the removal of our general partner because of the unitholders' dissatisfaction with its performance in managing our partnership will most likely result in the termination of the subordination period.

Unitholders' voting rights are restricted by the Partnership Agreement provision. It states that any units held by a person who owns 20% or more of any class of units then outstanding, other than our general partner, its affiliates, their transferees and persons who acquired such units with the prior approval of the board of directors of our general partner, cannot be voted on any matter. In addition, the Partnership Agreement contains provisions limiting the ability of unitholders to call meetings or to acquire information about our operations, as well as other provisions limiting the unitholders' ability to influence the manner or direction of management.

These provisions may discourage a person or group from attempting to remove our general partner or otherwise change our management. As a result of these provisions, the price at which the common units will trade may be lower because of the absence or reduction of a takeover premium in the trading price.

The control of our general partner may be transferred to a third party, and that party could replace our current management team, in each case without unitholder consent.

Our general partner may transfer its general partner interest to a third party in a merger, or in a sale of all or substantially all of its assets, without the consent of the unitholders. Furthermore, there is no restriction in the Partnership Agreement on the ability of the owners of our general partner from transferring their ownership interest in our general partner to a third party. The new owner of our general partner would then be in a position to replace the board of directors and officers of our general partner with its own choices, and to control the decisions taken by the board of directors and officers.

Our general partner's absolute discretion in determining the level of cash reserves may adversely affect our ability to make cash distributions to our unitholders.

Our Partnership Agreement requires our general partner to deduct from operating surplus cash reserves that, in its reasonable discretion, are necessary to fund our future operating expenditures. In addition, the Partnership Agreement permits our general partner to reduce available cash by establishing cash reserves for the proper conduct of our business, to comply with applicable law or agreements to which we are a party, or to provide funds for future distributions to partners. These cash reserves will affect the amount of cash available for distribution to our unitholders.

We do not have any employees and rely solely on employees of MarkWest Hydrocarbon and its affiliates who serve as our agents.

MarkWest Hydrocarbon and its affiliates conduct businesses and activities of their own in which we have no economic interest. If these separate activities are significantly greater than our activities, there could be material competition for the time and effort of the employees who provide services to our general partner. If the employees of MarkWest Hydrocarbon and its affiliates do not devote sufficient attention to the management and operation of our business, our financial results may suffer, and our ability to make distributions to our unitholders may be reduced.

We may issue additional common units without your approval, which would dilute your ownership interests.

During the subordination period, our general partner, without the approval of our unitholders, may cause us to issue up to 1,207,500 additional common units. Our general partner, without unitholder approval, may also cause us to issue an unlimited number of additional common units or other equity securities of equal rank with the common units, in several circumstances. These include:

•: the issuance of common units in connection with acquisitions or capital improvements that increase cash flow from operations per unit on a pro forma basis;
•: the conversion of subordinated units into common units;
•: the conversion of units of equal rank with the common units into common units under some circumstances;
•: the conversion of the general partner interest and the incentive distribution rights into common units as a result of the withdrawal of our general partner;
•: issuances of common units under our long-term incentive plan; or
•: issuances of common units to repay indebtedness, the cost of servicing which is greater than the distribution obligations associated with the units issued in connection with the debt's retirement.

The issuance of additional common units or other equity securities of equal or senior rank will have the following effects:

•: our unitholders' proportionate ownership interest in us will decrease;
•: the amount of cash available for distribution on each unit may decrease;
•: because a lower percentage of total outstanding units will be subordinated units, the risk that a shortfall in the payment of the minimum quarterly distribution will be borne by our common unitholders will increase;
•: the relative voting strength of each previously outstanding unit may be diminished;
•: the market price of the common units may decline; and

•: the ratio of taxable income to distributions may increase.

After the end of the subordination period, we may issue an unlimited number of limited partner interests of any type without the approval of our unitholders. Our Partnership Agreement does not give our unitholders the right to approve our issuance of equity securities ranking junior to the common units at any time.

Our general partner has a limited call right that may require you to sell your common units at an undesirable time or price.

If at any time more than 80% of the outstanding common units are owned by our general partner and its affiliates, our general partner will have the right, but not the obligation, which it may assign to any of its affiliates or to us, to acquire all, but not less than all, of the remaining common units held by unaffiliated persons at a price not less than their then-current market price. As a result, you may be required to sell your common units at an undesirable time or price and may not receive any return on your investment. You may also incur a tax liability upon a sale of your units.

You may not have limited liability if a court finds that unitholder action constitutes control of our business.

Under Delaware law, you could be held liable for our obligations as a general partner if a court determined that the right or the exercise of the right by our unitholders as a group to remove or replace our general partner, to approve some amendments to the Partnership Agreement, or to take other action under our Partnership Agreement was considered participation in the "control" of our business.

Our general partner usually has unlimited liability for our obligations, such as its debts and environmental liabilities, except for those of our contractual obligations that are expressly made without recourse to our general partner.

In addition, Section 17-607 of the Delaware Revised Uniform Limited Partnership Act provides that, under some circumstances, a unitholder may be liable to us for the amount of a distribution for a period of three years from the date of the distribution.

Tax Risks to Common Unitholders

In addition to reading the following risk factors, you should read "Material Tax Consequences" for a more complete discussion of the expected material federal income tax consequences of owning and disposing of common units.

Our tax treatment depends on our status as a partnership for federal income tax purposes, as well as our not being subject to entity-level taxation by individual states. If the IRS were to treat us as a corporation or if we become subject to entity-level taxation for state tax purposes, it would reduce the amount of cash available for distribution to you.

The anticipated after-tax economic benefit of an investment in our common units depends largely on our being treated as a partnership for federal income tax purposes. We have not requested, and do not plan to request, a ruling from the IRS on this or any other tax matter affecting us.

If we were treated as a corporation for federal income tax purposes, we would pay federal income tax on our taxable income at the corporate tax rate, which is currently a maximum of 35%, and would likely pay state income tax at varying rates. Distributions to you would generally be taxed again as corporate distributions, and no income, gains, losses or deductions would flow through to you. Because a tax would be imposed upon us as a corporation, our cash available for distribution to you would be substantially reduced. Therefore, treatment of us as a corporation would result in a material reduction

in the anticipated cash flow and after-tax return to the unitholders, likely causing a substantial reduction in the value of our common units.

Current law may change so as to cause us to be treated as a corporation for federal income tax purposes or otherwise subject us to entity-level taxation. In addition, because of widespread state budget deficits and other reasons, several states are evaluating ways to subject partnerships to entity-level taxation through the imposition of state income, franchise and other forms of taxation. For example, we will be subject to a new entity level tax on the portion of our income that is generated in Texas beginning in our tax year that ends December 31, 2007. Specifically, the Texas margin tax will be imposed at a maximum effective rate of 1.0% of our gross income apportioned to Texas. Imposition of such a tax on us by Texas, or any other state, will reduce the cash available for distribution to you.

Our partnership agreement provides that if a law is enacted or existing law is modified or interpreted in a manner that subjects us to taxation as a corporation or otherwise subjects us to entity-level taxation for federal, state or local income tax purposes, the minimum quarterly distribution amount and the target distribution amounts will be adjusted to reflect the impact of that law on us.

If the IRS contests the federal income tax positions we take, the market for our common units may be adversely impacted and the cost of any IRS contest will reduce our cash available for distribution to you.

We have not requested a ruling from the IRS with respect to our treatment as a partnership for federal income tax purposes or any other matter affecting us. The IRS may adopt positions that differ from the conclusions of our counsel expressed in this prospectus or from the positions we take. It may be necessary to resort to administrative or court proceedings to sustain some or all of our counsel's conclusions or the positions we take. A court may not agree with some or all of our counsel's conclusions or positions we take. Any contest with the IRS may materially and adversely impact the market for our common units and the price at which they trade. In addition, our costs of any contest with the IRS will be borne indirectly by our unitholders and our general partner because the costs will reduce our cash available for distribution.

You may be required to pay taxes on your share of our income even if you do not receive any cash distributions from us.

Because our unitholders will be treated as partners to whom we will allocate taxable income which could be different in amount than the cash we distribute, you will be required to pay any federal income taxes and, in some cases, state and local income taxes on your share of our taxable income even if you receive no cash distributions from us. You may not receive cash distributions from us equal to your share of our taxable income or even equal to the actual tax liability that results from that income.

Tax gain or loss on disposition of our common units could be more or less than expected.

If you sell your common units, you will recognize a gain or loss equal to the difference between the amount realized and your tax basis in those common units. Prior distributions to you in excess of the total net taxable income you were allocated for a common unit, which decreased your tax basis in that common unit, will, in effect, become taxable income to you if the common unit is sold at a price greater than your tax basis in that common unit, even if the price you receive is less than your original cost. A substantial portion of the amount realized, whether or not representing gain, may be ordinary income. In addition, if you sell your units, you may incur a tax liability in excess of the amount of cash you receive from the sale.

Tax-exempt entities and foreign persons face unique tax issues from owning our common units that may result in adverse tax consequences to them.

Investment in common units by tax-exempt entities, such as individual retirement accounts (known as IRAs), and non-U.S. persons raises issues unique to them. For example, virtually all of our income allocated to organizations that are exempt from federal income tax, including individual retirement accounts and other retirement plans, will be unrelated business taxable income and will be taxable to them. Distributions to non-U.S. persons will be reduced by withholding taxes at the highest applicable effective tax rate, and non-U.S. persons will be required to file United States federal tax returns and pay tax on their share of our taxable income.

We will treat each purchaser of common units as having the same tax benefits without regard to the actual common units purchased. The IRS may challenge this treatment, which could adversely affect the value of the common units.

Because we cannot match transferors and transferees of common units and because of other reasons, we will adopt depreciation and amortization positions that may not conform to all aspects of existing Treasury Regulations. A successful IRS challenge to those positions could adversely affect the amount of tax benefits available to you. It also could affect the timing of these tax benefits or the amount of gain from your sale of common units and could have a negative impact on the value of our common units or result in audit adjustments to your tax returns. Please read "Material Tax Consequences—Tax Consequences of Unit Ownership—Section 754 Election" for a further discussion of the effect of the depreciation and amortization positions we adopted.

The sale or exchange of 50% or more of our capital and profits interests during any twelve-month period will result in the termination of our partnership for federal income tax purposes.

We will be considered to have terminated for federal income tax purposes if there is a sale or exchange of 50% or more of the total interests in our capital and profits within a twelve-month period. Our termination would, among other things, result in the closing of our taxable year for all unitholders and could result in a deferral of depreciation deductions allowable in computing our taxable income. Please read "Material Tax Consequences—Disposition of Common Units—Constructive Termination" for a discussion of the consequences of our termination for federal income tax purposes.

You will likely be subject to state and local taxes and return filing requirements in states where you do not live as a result of investing in our common units.

In addition to federal income taxes, you will likely be subject to other taxes, including foreign, state and local taxes, unincorporated business taxes and estate, inheritance or intangible taxes that are imposed by the various jurisdictions in which we do business or own property, even if you do not live in any of those jurisdictions. You will likely be required to file foreign, state and local income tax returns and pay state and local income taxes in some or all of these various jurisdictions. Further, you may be subject to penalties for failure to comply with those requirements. We currently do business or own property in nine states, most of which impose income taxes. As we make acquisitions or expand our business, we may own assets or conduct business in additional states that impose a personal income tax. It is your responsibility to file all United States federal, foreign, state and local tax returns. Our counsel has not rendered an opinion on the state or local tax consequences of an investment in our common units.

USE OF PROCEEDS

We will not receive any proceeds from the sale of common units by the selling unitholders.

PRICE RANGE OF COMMON UNITS AND DISTRIBUTIONS

Our common units have been listed on the American Stock Exchange ("AMEX"), under the symbol "MWE." The following table sets forth the high and low sales prices of the common units as reported by AMEX, as well as the amount of cash distributions paid per quarter for 2006, 2005 and 2004 and subsequent interim periods.

Quarter Ended	High		Low		Distributions Per Unit(1)		Record Date	Payment Date
Through December 4, 2006	$	57.99	$	46.75		—	—	—
September 30, 2006	$	49.50	$	41.00	$	0.97	November 3, 2006	November 14, 2006
June 30, 2006	$	46.65	$	39.50	$	0.92	August 7, 2006	August 14, 2006
March 31, 2006	$	47.99	$	43.51	$	0.87	May 5, 2006	May 15, 2006
December 31, 2005	$	50.95	$	42.01	$	0.82	February 8, 2006	February 14, 2006
September 30, 2005	$	53.50	$	47.18	$	0.82	November 8, 2005	November 14, 2005
June 30, 2005	$	51.54	$	46.51	$	0.80	August 9, 2005	August 15, 2005
March 31, 2005	$	52.50	$	45.25	$	0.80	May 10, 2005	May 16, 2005
December 31, 2004	$	48.69	$	42.50	$	0.78	February 2, 2005	February 11, 2005
September 30, 2004	$	45.80	$	37.73	$	0.76	November 3, 2004	November 12, 2004
June 30, 2004	$	40.07	$	33.50	$	0.74	July 30, 2004	August 13, 2004
March 31, 2004	$	41.66	$	37.70	$	0.69	April 30, 2004	May 14, 2004

(1): Includes both common units and subordinated units.

We have also issued 3,000,000 subordinated units, for which there is no established public-trading market. Pursuant to the terms of the partnership agreement, 2,400,000 of these units were converted into common units during 2005 and 2006. 600,000 subordinated units were outstanding as of December 4, 2006. There was one holder of record of our subordinated units as of December 4, 2006.

The last reported sale price of the common units on the American Stock Exchange on December 4, 2006 was $57.06. As of December 4, 2006, there were 139 holders of record of our common units.

UNAUDITED PRO FORMA FINANCIAL STATEMENTS

On March 31, 2005, we acquired a 50% non-operating membership interest in Starfish Pipeline Company, LLC (the "Starfish acquisition") from an affiliate of Enterprise Products Partners, L.P. for $41.7 million. The acquisition was financed through our credit facility. On November 1, 2005, we acquired 100% of the equity interests in Javelina Company, Javelina Pipeline Company, Javelina Land Company, L.L.C. and related entities (collectively, "Javelina") for $357.0 million, plus $41.3 million for net working capital. We had a net draw of $386.5 million on our credit facility to finance the Javelina acquisition. We also incurred $10.4 million of debt issuance costs, of which $5.4 million was paid in cash and $5.0 million was paid from drawings on the credit facility. The final determination of the working capital amount and allocation of the purchase price has not been completed.

The following unaudited pro forma consolidated statement of operations for the year ended December 31, 2005 and the nine months ended September 30, 2006 gives effect to the following transactions as if they all occurred on January 1, 2005:

•: the Javelina acquisition;
•: our net borrowings under our credit facility to finance the Javelina acquisition;
•: the Starfish acquisition;
•: our net borrowings on our credit facility to partially finance the Starfish acquisition;
•: our November 2005 private placement of 1,644,065 common units at an offering price of $44.21 per unit, and a capital contribution from our general partner to maintain its 2% general partner interest;
•: our December 2005 private placement of 574,714 common units at an offering price of $43.50 per unit, and a capital contribution from our general partner to maintain its 2% general partner interest;
•: our July 2006 public offering of 3,300,000 common units at a public offering price of $39.75, and a capital contribution from our general partner to maintain its 2% general partner interest;
•: our July 2006 and October 2006 private placements of $275 million in aggregate principal amount of 8¹/2% Senior Notes due 2016 to qualified institutional buyers;
•: the application of the net proceeds from our November 2005 private placement, December 2005 private placement and our June 2006 public offering of common units, including, in each case, the contribution from our general partner, and the application of the net proceeds from our July 2006 and October 2006 private placements of 8¹/2% Senior Notes due 2016 to repay borrowings under our credit facility incurred in connection with the acquisition of both Javelina and Starfish.

These transaction adjustments are presented and discussed in the notes to the unaudited pro forma financial statements. You should read the unaudited pro forma financial statements and accompanying notes along with our historical financial statements and the accompanying notes and the historical financial statements of Javelina and Starfish and the accompanying notes incorporated by reference into this prospectus.

The information presented under the heading "Javelina" in the pro forma financial statements reflects the results of operations of Javelina Company and Javelina Pipeline Company for the ten months ended October 31, 2005. No separate information for the other Javelina entities has been presented as the amounts are immaterial. In addition to property, plant and equipment, the acquisition of equity interests resulted in the assumption of certain current assets and liabilities of the companies as of November 1, 2005. Our historical consolidated statements of operations for the year ended December 31, 2005 and the nine months ended September 30, 2006 include results of Javelina from November 1, 2005 through December 31, 2005 and from January 1, 2006 through September 30, 2006,

respectively. Accordingly, the separate Javelina information only includes the ten months ended October 31, 2005 that are not already included in our results.

The information presented under the heading "Starfish" reflects the results of operations of Starfish Pipeline Company, LLC for the three months ended March 31, 2005. Our historical consolidated statements of operations for the year ended December 31, 2005 and the nine months ended September 30, 2006 include the Starfish acquisition from April 1, 2005 through December 31, 2005 and from January 1, 2006 through September 30, 2006, respectively. Accordingly, the separate Starfish information reflects only the three months ended March 31, 2005, that are not already included in our results.

The pro forma statements of operations were derived by adjusting the historical financial statements of MarkWest Energy Partners, L.P. The adjustments are based on currently available information. We believe, however, that the adjustments provide a reasonable basis for presenting the significant effects of the transactions described above. The unaudited pro forma financial statements do not purport to present our results of operations had the acquisitions or the other transactions actually been completed as of the dates indicated. Moreover, the statements do not project our financial position or results of operations for any future date or period.

			Twelve Months Ended December 31, 2005													Nine Months Ended September 30, 2006
			MarkWest Energy			Starfish		Javelina		Pro Forma Adjustments			Pro Forma As Adjusted			MarkWest Energy			Pro Forma Adjustments			Pro Forma As Adjusted
			(in thousands, except per unit amounts)
Revenues:
	Sales to unaffiliated parties		$	434,162		$	—	$	255,864	$	—		$	690,026		$	393,482		$	—		$	393,482
	Sales to affiliate			64,922			—		—		—			64,922			55,288			—			55,288
Derivatives				—			—		—		—			—			6,009			—			6,009

		Total Revenues		499,084			—		255,864		—			754,948			454,779			—			454,779

Operating expenses:
	Purchased product costs			366,878			—		178,147		—			545,025			258,791			—			258,791
	Facility expenses			47,972			—		42,613		—			90,585			44,964			—			44,964
	Selling, general and administrative expenses			21,573			—		1,363		—			22,936			30,404			—			30,404
	Depreciation			19,534			—		7,677		(964	)(a)		26,247			22,462			—			22,462
	Amortization of intangible assets			9,656			—		—		6,405	(b)		16,061			12,072			—			12,072
	Accretion of asset retirement and lease obligations			159			—		—		—			159			75			—			75

		Total operating expenses		465,772			—		229,800		5,441			701,013			368,768			—			368,768

		Income from operations		33,312			—		26,064		(5,441	)		53,935			86,011			—			86,011
Other income (expense):
	Earnings in unconsolidated affiliates			(2,153	)		984		—		(72	)(c)		(1,241	)		3,240			—			3,240
	Interest income			367			—		354		—			721			709						709
	Interest expense			(22,469	)		—		—		(24,060 (750 6,518 9,546 (4,456	)(d) )(e) (f) (g) )(h)		(35,672	)		(31,213	)		4,773 (2,515	(g) )(h)		(28,955	)
	Amortization of deferred financing costs			(6,780	)		—		—		(4,926 (10,436	)(d) )(h)		(22,142	)		(7,700	)		1,450	(h)		(6,250	)
	Miscellaneous income (expense)			78			—		—		—			78			7,577			—			7,577

	Income before Texas Margin tax			2,355			984		26,418		(34,077	)		(4,320	)		58,624			3,708			62,332
	Texas Margin tax			—			—		—		—			—			(679	)		—			(679	)

Net income (loss)			$	2,355		$	984	$	26,418	$	(34,077	)	$	(4,320	)	$	57,945		$	3,708		$	61,653

Interest in net income:
	General partner	$	2,113	$	1,980		$	448	$	522
	Limited partners	$	242	$	(6,300	)	$	57,497	$	61,131
Net income per limited partner unit:
	Basic	$	0.02	$	(0.38	)	$	3.95	$	3.29
	Diluted	$	0.02	$	(0.38	)	$	3.93	$	3.28

		Twelve Months Ended December 31, 2005					Nine Months Ended September 30, 2006
		MarkWest Energy	Starfish	Javelina	Pro Forma Adjustments	Pro Forma As Adjusted	MarkWest Energy	Pro Forma Adjustments	Pro Forma As Adjusted
		(in thousands)
Weighted average units outstanding
	Basic	10,895			5,519	16,414	13,910	3,300	17,210
	Diluted	10,929			5,519	16,448	13,975	3,300	17,275

(a): The pro forma adjustment to Javelina depreciation expense for the year ended December 31, 2005 are as follows (in thousands):

Historical depreciation	$	(7,677	)
Pro forma depreciation(1)		6,713

Pro forma adjustment to depreciation expense		(964	)

(1): Pro forma depreciation is based on the following lives: Buildings (40 years), Pipeline and plant equipment (20 years), Other (3 years).
(b): Amortization of identifiable intangible assets and customer contracts of $192.1 million for the year ended December 31, 2005, over the estimated useful life of 25 years.
(c): Amortization of the excess Starfish purchase price over net book value of $4.9 million for the three months from January 1, 2005, to the Starfish acquisition date, over the estimated useful life of 17 years.
(d): Interest expense on the debt associated with the Javelina acquisition of $386.5 million for the first 10 months of the year ended December 31, 2005, at a weighted average interest of 7.50%. The effect of a 0.125% variance in annual interest rates under our credit facility on pro forma interest expense would have been approximately $0.4 million. Amortized deferred financing costs for the nine months ended September 30, 2005, based upon the term of the debt facility, and expensed debt issuance costs were both associated with the debt for the Javelina acquisition.
(e): Interest expense on the incremental debt associated with the Starfish acquisition of $40.0 million for the three months from January 1, 2005, to the Starfish acquisition date, at a weighted average interest rate of 7.50%. The effect of a 0.125% variance in annual interest rates under our credit facility on pro forma interest expense would have been approximately $12,500.
(f): Reduction of interest expense resulting from the application of the net proceeds from our November 2005 private placement and December 2005 private placements of $74.1 million and $25.4 million, respectively.
(g): Reduction of interest expense resulting from the application of the estimated net proceeds from our July 2006 public equity offering of $127.3 million.
(h): Adjusted to reflect our July 2006 and October 2006 private placement of 8¹/2% Senior Notes due 2016. The twelve months ended December 31, 2005, reflect additional interest expense of $4.5 million and additional amortization of deferred financing costs ($0.6 million associated with the notes in our July 2006 private placement, $3.3 million reduction associated with retirement of existing debt, and $13.1 million acceleration associated with existing debt). The nine months ended September 30, 2006, reflect additional interest expense of $2.5 million and a reduction in deferred financing costs ($0.3 million associated with the notes in our July 2006 private placement, and $1.8 million associated with existing debt).

SELECTED HISTORICAL FINANCIAL INFORMATION

The following table shows selected historical financial and operating data of the MarkWest Hydrocarbon Midstream Business and us as of and for the periods indicated. The MarkWest Hydrocarbon Midstream Business represents substantially all of MarkWest Hydrocarbon's historical natural gas gathering and processing and NGL transportation, fractionation and storage businesses prior to our formation. The selected historical financial data for the MarkWest Hydrocarbon Midstream Business as of and for the year ended December 31, 2001 is derived from the audited financial statements of the MarkWest Hydrocarbon Midstream Business. Our selected historical financial data as of and for the years ended December 31, 2002, 2003, 2004 and 2005 is derived from our audited financial statements.

The historical financial statements for all periods prior to our formation differ substantially from our financial statements and unaudited pro forma financial statements, principally because of the contracts we entered into with MarkWest Hydrocarbon at the closing of our initial public offering. The largest of these differences is in revenues and purchased product costs. Historically, revenues and purchased product costs in the MarkWest Hydrocarbon Midstream Business were higher because:

•: its revenues included the aggregate sales price for all the NGL products produced in its operations; and
•: its purchased product costs included the cost of natural gas purchases needed to replace the Btu content of the NGLs extracted in its processing operations under keep-whole contracts and the percentage of the proceeds from the sale of NGL products remitted to producers under percent-of-proceeds contracts.

In contrast, after entering into contractual arrangements with MarkWest Hydrocarbon in connection with our initial public offering:

•: our revenues related to these assets include only the fees we receive for processing natural gas, transporting, fractionating and storing NGLs and the aggregate proceeds from NGL sales we receive under our percent-of-proceeds contracts; and
•: our purchased product costs related to these assets primarily consist of the percentage of proceeds from the sale of NGL products remitted to producers under our percent-of-proceeds contracts, with a small portion of our purchased product costs attributable to natural gas purchase to satisfy our obligations under our keep-whole contracts.

Sustaining capital expenditures are capital expenditures made to replace partially or fully depreciated assets in order to maintain the existing operating capacity of our assets and to extend their useful lives. Expansion capital expenditures are capital expenditures made to expand the existing operating capacity of our assets, whether through construction or acquisition. We treat repair and maintenance expenditures that do not extend the useful life of existing assets as facility expenses as we incur them.

We derived the information in the following table from, and that information should be read together with and is qualified in its entirety by reference to, the historical and pro forma financial statements and the accompanying notes included elsewhere in this prospectus or incorporated by reference herein. The table should be read together with "Item 7. Management's Discussion and Analysis of Financial Condition and Results of Operations" incorporated by reference from our Annual Report on Form 10-K for the year ended December 31, 2005, as amended, and "Item 2. Management's Discussion and Analysis of Financial Condition and Results of Operations" incorporated by reference from our Quarterly Report on Form 10-Q for the three months ended September 30, 2006.

		MarkWest Hydrocarbon Midstream Business			Partnership
		Year Ended December 31,															Nine Months Ended September 30,
		2001			2002			2003(a)			2004(b)			2005(c)			2005			2006
		(in thousands, except per unit amounts)
Statement of Operations:
Revenues		$	93,675		$	70,246		$	117,430		$	301,314		$	499,084		$	323,165		$	454,779
Operating expenses:
	Purchased product costs		65,483			38,906			70,832			211,534			366,878			233,521			258,791
	Facility expenses		13,138			15,101			20,463			29,911			47,972			33,205			44,964
	Selling, general and administrative expenses		5,047			5,411			8,598			16,133			21,573			16,487			30,404
	Depreciation and amortization of intangible assets		4,490			4,980			7,548			19,196			29,190			19,961			34,534
	Impairments		—			—			1,148			130			—			—			—
	Accretion of asset retirement obligation		—			—			—			13			159			136			75

Total operating expenses			88,158			64,398			108,589			276,917			465,772			303,310			368,768

Income from operations			5,517			5,848			8,841			24,397			33,312			19,855			86,011
	Interest expense, net		(1,307	)		(1,414	)		(4,057	)		(14,385	)		(28,882	)		(14,440	)		(38,204	)
	Earnings (loss) from unconsolidated affiliates		—			—			—			(65	)		(2,153	)		(9	)		3,240
	Miscellaneous income (expense)		—			52			(25	)		15			78			132			7,577

Income before income taxes			4,210			4,486			4,759			9,962			2,355			5,538			58,624
	Provision (benefit) for income taxes		1,624			(17,175	)		—			—			—			—			679

Net income		$	2,586		$	21,661		$	4,759		$	9,962		$	2,355		$	5,538		$	57,945

Net income per limited partner unit:
	Basic	$	0.86		$	4.86		$	0.95		$	1.31		$	0.02		$	0.45		$	3.95
	Diluted	$	0.86		$	4.83		$	0.94		$	1.31		$	0.02		$	0.45		$	3.93
Cash distributions declared per limited partner unit			—		$	1.23		$	2.47		$	2.97		$	3.28		$	2.42		$	2.76

	MarkWest Hydrocarbon Midstream Business		Partnership
	As of December 31,										As of September 30,
	2001		2002		2003(a)		2004(b)		2005(c)		2006
	(in thousands)
Balance Sheet Data (at period end):
Working capital	$	18,240	$	1,762	$	2,457	$	10,547	$	11,944	$	6,716
Property, plant and equipment, net		82,008		79,824		184,214		280,635		492,961		514,819
Total assets		104,891		87,709		212,871		529,422		1,046,093		1,061,974
Total long-term debt		19,179		21,400		126,200		225,000		604,000		479,654
Partners' capital		65,429		60,863		64,944		241,142		307,175		455,223

			MarkWest Hydrocarbon Midstream Business			Partnership
			Year Ended December 31,															Nine Months Ended September 30,
			2001			2002			2003(a)			2004(b)			2005(c)			2005			2006
			(in thousands)
Cash Flow Data:
Net cash flow provided by (used in):
	Operating activities		$	(524	)	$	33,502		$	21,229		$	42,275		$	42,090		$	25,631		$	120,508
	Investing activities			(8,997	)		(2,056	)		(112,893	)		(273,176	)		(469,308	)		(91,834	)		(66,554	)
	Financing activities			9,521			(28,670	)		97,641			246,411			423,060			51,985			(43,460	)
Other Financial Data:
Sustaining capital expenditures(d)			$	576		$	511		$	1,041		$	1,163		$	2,181		$	1,322		$	1,451
Expansion capital expenditures(d)				9,075			1,634			1,903			29,304			68,569			48,861			41,514

		Total capital expenditures	$	9,651		$	2,145		$	2,944		$	30,467		$	70,750		$	50,183		$	42,965

Net operating margin(e)			$	28,192		$	31,340		$	46,598		$	89,780		$	132,206		$	89,644		$	195,988
EBITDA(f)			$	10,007		$	10,885		$	16,378		$	43,630		$	60,794		$	40,149		$	131,623

		MarkWest Hydrocarbon Midstream Business	Partnership
		Year Ended December 31,					Nine Months Ended September 30,
		2001	2002	2003(a)	2004(b)	2005(c)	2005	2006
Operating Data:
Southwest:
East Texas
	Gathering systems throughput (Mcf/d)	—	—	—	259,300	321,000	313,000	371,000
	NGL product sales (gallons)	—	—	—	41,478,000	126,476,000	88,958,000	117,912,000
Oklahoma
	Foss Lake gathering systems throughput (Mcf/d)	—	—	57,000	60,900	75,800	73,000	86,000
	Arapaho NGL product sales (gallons)	—	—	2,910,000	45,273,000	60,903,000	46,180,000	57,586,000
Other
	Appleby gathering systems throughput (Mcf/d)	—	—	23,800	27,100	33,400	33,000	34,000
	Other gathering systems throughput (Mcf/d)	—	—	20,500	17,000	16,500	16,000	20,000
	Lateral throughput volumes (Mcf/d)(g)	—	—	32,100	75,500	81,000	90,000	84,000
Gulf Coast:
	Natural gas processed for a fee (Mcf/d)	—	—	—	—	115,000	NA	125,000
	NGLs fractionated for a fee (Gal/day)	—	—	—	—	816,000	NA	1,090,000

Northeast:
Appalachia:
	Natural gas processed for a fee (Mcf/d)	192,000	202,000	202,000	203,000	197,000	197,000	200,000
	NGLs fractionated for a fee (Gal/day)	423,000	476,000	458,000	475,000	430,000	426,000	451,000
	NGL product sales (gallons)	—	38,813,000	40,305,000	42,105,000	41,700,000	31,051,000	32,226,000


Michigan:
	Natural gas processed for a fee (Mcf/d)	8,800	13,800	15,000	12,300	6,600	6,700	6,500
	Crude oil transported for a fee (Bbl/d)	—	—	15,100	14,700	14,200	14,100	14,600
	NGL product sales (gallons)	8,000,000	11,100,000	11,800,000	9,818,000	5,697,000	4,447,000	4,344,000

(a): We acquired our Foss Lake gathering system in December 2003. We acquired our Arapaho processing plant in December 2003. We acquired our Pinnacle gathering systems in late March 2003. We acquired our Lubbock pipeline in September 2003 and our Michigan Crude Pipeline in December 2003.
(b): We acquired our East Texas System in late July 2004. We acquired our Hobbs lateral pipeline in April 2004.
(c): We completed our investment in Starfish on March 31, 2005, and acquired Javelina on November 1, 2005.
(d): Sustaining capital expenditures include expenditures to replace partially or fully depreciated assets in order to maintain the existing operating capacity of our assets and to extend their useful lives. Expansion capital includes expenditures made to expand or increase the efficiency of the existing operating capacity of our assets. Expansion capital expenditures include expenditures that facilitate an increase in volumes within our operations, whether through construction or acquisition.
(e): Management evaluates contract performance on the basis of net operating margin (a "non-GAAP" financial measure), which is defined as income (loss) from operations, excluding facility expenses, selling, general and administrative expense, depreciation, amortization, impairments and accretion of asset retirement obligations. These charges have been excluded for the purpose of enhancing the understanding, by both management and investors, of the underlying baseline operating performance of our contractual arrangements, which management uses to evaluate our financial performance, for purposes of planning and forecasting future periods. Net operating margin does not have any standardized definition and, therefore, is unlikely to be comparable to similar measures presented by other reporting companies. Net operating margin results should not be evaluated in isolation of, or as a substitute for, our financial results prepared in accordance with GAAP. Our use of net operating margin, and the underlying methodology in excluding certain charges, is not necessarily an indication of the results of operations that may be expected in the future, or that we will not, in fact, incur such charges

in future periods. The following reconciles non-GAAP financial measure to income from operations, the most directly comparable GAAP financial performance measure:

		Year Ended December 31,										Nine Months Ended September 30,
		2001		2002		2003		2004		2005		2005		2006
		(in thousands)
Revenues		$	93,675	$	70,246	$	117,430	$	301,314	$	499,084	$	323,165	$	454,779
Purchased product costs			65,483		38,906		70,832		211,534		366,878		233,521		258,791

Net operating margin			28,192		31,340		46,598		89,780		132,206		89,644		195,988
	Facility expenses		13,138		15,101		20,463		29,911		47,972		33,205		44,964
	Selling, general and administrative expenses		5,047		5,411		8,598		16,133		21,573		16,487		30,404
	Depreciation		4,490		4,980		7,548		15,556		19,534		13,673		22,462
	Amortization of intangible assets		—		—		—		3,640		9,656		6,288		12,072
	Accretion of asset retirement obligation		—		—		—		13		159		136		75
	Impairments		—		—		1,148		130		—		—		—

Income from operations		$	5,517	$	5,848	$	8,841	$	24,397	$	33,312	$	19,855	$	86,011

(f): EBITDA is defined as earnings before income taxes, plus depreciation and amortization expense and interest expense. EBITDA (i) is not a measure of performance calculated in accordance with GAAP, and (ii) should not be considered in isolation or as a substitute for net income, income from operations or cash flow as reflected in our financial statements.

EBITDA is presented because such information is relevant and is used by management, industry analysts, investors, lenders and rating agencies to assess the financial performance and operating results of our fundamental business activities. Management believes that the presentation of EBITDA is useful to lenders and investors because of its use in the midstream natural gas industry and for master limited partnerships as an indicator of the strength and performance of our ongoing business operations. Additionally, management believes that EBITDA provides additional and useful information to our investors for trending, analyzing and benchmarking our operating results from period to period as compared to other companies that may have different financing and capital structures. The presentation of EBITDA allows investors to view our performance in a manner similar to the methods used by management and provides additional insight to our operating results.

EBITDA is used by management to determine our operating performance and along with other data as internal measures for setting annual operating budgets, assessing financial performance of our numerous business locations, evaluating targeted businesses for acquisition and determining incentive compensation. We have a number of business locations located in different regions of the United States. EBITDA can be a meaningful measure of financial performance because it excludes factors which are outside the control of the employees responsible for operating and managing the business locations, and provides information management can use to evaluate the performance of the business locations, or the region where they are located, and the employees responsible for operating them.

There are material limitations to using a measure such as EBITDA, including the difficulty associated with using it as the sole measure to compare the results of one company to another, and the inability to analyze certain significant items that directly affect a company's net income or loss. Management compensates for these limitations by considering EBITDA in conjunction with its analysis of other GAAP financial measures, such as gross profit, net income and cash flow from operating activities. In

addition, our calculation of EBITDA may not be consistent with similarly titled measures of other companies and should be viewed in conjunction with measurements that are computed in accordance with GAAP. EBITDA for the periods described herein is calculated in the same manner as presented by us in the past. A reconciliation of EBITDA to net income is presented below.

The following table reconciles EBITDA with our net income:

	Year Ended December 31,											Nine Months Ended September 30,
	2001		2002			2003		2004		2005		2005		2006
	(in thousands)
Net income	$	2,586	$	21,661		$	4,759	$	9,962	$	2,355	$	5,538	$	57,497
Interest expense		1,307		1,419			4,071		14,472		29,249		14,650		38,913
Taxes		1,624		(17,175	)		—		—		—		—		679
Depreciation and amortization		4,490		4,980			7,548		19,196		29,190		19,961		34,534

EBITDA	$	10,007	$	10,885		$	16,378	$	43,630	$	60,794	$	40,149	$	131,623

(g): The Lubbock and Hobbs pipelines are the only lateral pipelines we own that produce revenue on a volumetric basis. We receive a flat fee for the use of our other lateral pipelines, and consequently, the throughput volume data from these other lateral pipelines is excluded from this data.

CASH DISTRIBUTION POLICY

Distributions of Available Cash

General. Within approximately 45 days after the end of each quarter, we distribute all of our available cash to unitholders of record on the applicable record date.

Definition of Available Cash. We define available cash in the glossary, and it generally means, for each fiscal quarter:

•

all cash on hand at the end of the quarter;

•

less the amount of cash that our general partner determines in its reasonable discretion is necessary or appropriate to:

•: provide for the proper conduct of our business;
•: comply with applicable law, any of our debt instruments, or other agreements; or
•: provide funds for distributions to our unitholders and to our general partner for any one or more of the next four quarters;

•

plus all cash on hand on the date of determination of available cash for the quarter resulting from working capital borrowings made after the end of the quarter. Working capital borrowings are generally borrowings that are made under our credit facility and in all cases are used solely for working capital purposes or to pay distributions to partners.

Minimum Quarterly Distribution. Common units are entitled to receive distributions from operating surplus of $0.50 per quarter, or $2.00 on an annualized basis, before any distributions are paid on our subordinated units. There is no guarantee that we will pay the minimum quarterly distribution on the common units in any quarter, and we are prohibited from making any distributions to unitholders if it would cause an event of default under our credit facility. As reflected below, our definition of operating surplus contains a $6.3 million basket. This basket is a provision that enables us, if we choose, to distribute as operating surplus up to $6.3 million of cash we receive in the future from non-operating sources, such as asset sales, issuances of securities and long-term borrowings, that would otherwise be distributed as capital surplus.

Contractual Restrictions on our Ability to Distribute Available Cash. Our ability to distribute available cash is contractually restricted by the terms of our credit facility. Our credit facility contains covenants requiring us to maintain certain financial ratios and a minimum net worth. We are prohibited from making any distribution to unitholders if such distribution would cause an event of default or otherwise violate a covenant under our credit facility. In addition, our credit facility prohibits us from borrowing more than $0.75 per outstanding unit during any consecutive 12-month period for the purpose of making distributions to our unitholders. Our credit facility provides that any amount so borrowed must be repaid once annually. Please read "Item 7. Management's Discussion and Analysis of Financial Condition and Results of Operations—Description of Credit Facility" as set forth in our most recent Annual Report on Form 10-K incorporated by reference herein.

Operating Surplus and Capital Surplus

General. All cash distributed to unitholders is characterized as either "operating surplus" or "capital surplus." We distribute available cash from operating surplus differently than available cash from capital surplus.

Definition of Operating Surplus. We define operating surplus in the glossary, and for any period it generally means:

•: $470,000 representing cash on hand at the closing of our initial public offering; plus
•: $6.3 million (as described above); plus
•: all of our cash receipts since the initial public offering, excluding cash from borrowings that are not working capital borrowings, sales of equity and debt securities and sales or other dispositions of assets outside the ordinary course of business; plus
•: working capital borrowings made after the end of a quarter but before the date of determination of operating surplus for the quarter; less
•: all of our operating expenditures since the initial public offering, including the repayment of working capital borrowings, but not the repayment of other borrowings, and including maintenance capital expenditures; and less
•: the amount of cash reserves that our general partner deems necessary or advisable to provide funds for future operating expenditures.

Definition of Capital Surplus. We also define capital surplus in the glossary, and it is generally generated only by:

•: borrowings other than working capital borrowings;
•: sales of debt and equity securities; and
•: sales or other disposition of assets for cash, other than inventory, accounts receivable and other current assets sold in the ordinary course of business or as part of normal retirements or replacements of assets.

Characterization of Cash Distributions. We treat all available cash distributed as coming from operating surplus until the sum of all available cash distributed since we began operations equals the operating surplus as of the most recent date of determination of available cash. We treat any amount distributed in excess of operating surplus, regardless of its source, as capital surplus. While we do not currently anticipate that we will make any distributions from capital surplus in the near term, we may determine that the sale or disposition of an asset or business owned or acquired by us may be beneficial to our unitholders. If we distribute to you the equity we own in a subsidiary or the proceeds from the sale of one of our businesses, such a distribution would be characterized as a distribution from capital surplus.

Subordination Period

General. During the subordination period, which we define below and in the glossary, the common units have the right to receive distributions of available cash from operating surplus in an amount equal to the minimum quarterly distribution of $0.50 per quarter, plus any arrearages in the payment of the minimum quarterly distribution on the common units from prior quarters, before any distributions of available cash from operating surplus may be made on the subordinated units. The purpose of the subordinated units is to increase the likelihood that during the subordination period there will be available cash to be distributed on the common units.

Definition of Subordination Period. We define the subordination period in the glossary. The subordination period will extend until the first day of any quarter beginning after June 30, 2009 that each of the following tests are met:

•: distributions of available cash from operating surplus on each of the outstanding common units and subordinated units equaled or exceeded the minimum quarterly distribution for each of the three consecutive, non-overlapping four-quarter periods immediately preceding that date;
•: the "adjusted operating surplus" (as defined below) generated during each of the three immediately preceding, non-overlapping four-quarter periods equaled or exceeded the sum of the minimum quarterly distributions on all of the outstanding common units and subordinated units during those periods on a fully diluted basis and the related distribution on the 2% general partner interest during those periods; and
•: there are no arrearages in payment of the minimum quarterly distribution on the common units.

Early Conversion of Subordinated Units. Before the end of the subordination period, a portion of the subordinated units may convert into common units on a one-for-one basis immediately after the distribution of available cash to the partners in respect of any quarter ending on or after:

•: June 30, 2007 with respect to 20% of the subordinated units; and
•: June 30, 2008 with respect to 20% of the subordinated units.

The early conversions will occur if at the end of the applicable quarter each of the following occurs:

•: distributions of available cash from operating surplus on each common unit and subordinated unit equaled or exceeded the minimum quarterly distribution for each of the three consecutive, non-overlapping four-quarter periods immediately preceding that date;
•: the adjusted operating surplus generated during each of the three consecutive, non-overlapping four-quarter periods immediately preceding that date equaled or exceeded the sum of the minimum quarterly distributions on all of the outstanding common units and subordinated units during those periods on a fully diluted basis and the related distribution on the 2% general partner interest during those periods; and
•: there are no arrearages in payment of the minimum quarterly distribution on the common units.

However, the early conversion of the second, third or fourth 20% of the subordinated units may not occur until at least one year following the early conversion of the first, second or third 20% of the subordinated units, as the case may be. On each of August 15, 2005 and August 15, 2006, after meeting the financial tests set forth in our partnership agreement for the first early conversion of the first and second 20% of the subordinated units, we completed the conversion, on a one-for-one basis, of 600,000 subordinated units into common units.

In addition, on November 14, 2005, we completed the conversion, on a one-for-one basis, of 600,000 subordinated units into common units as a result of satisfying the following conditions in our partnership agreement:

•: distributions of available cash from operating surplus on each common unit and subordinated unit equaled or exceeded $2.50 for each of the two consecutive, non-overlapping four-quarter periods immediately preceding that date;
•: the adjusted operating surplus generated during each of the two consecutive, non-overlapping four-quarter periods immediately preceding that date equaled or exceeded the sum of a distribution of $2.50 on all of the outstanding common units and subordinated units during those

periods on a fully diluted basis and the related distribution on the 2% general partner interest during those periods; and

•: there were no arrearages in payment of the minimum quarterly distribution on the common units.

This additional early conversion was a one time occurrence.

In addition to the early conversion of subordinated units described above, 20% of the subordinated units to converted into common units on a one-for-one basis on November 15, 2006 as a result of satisfying the following conditions in our partnership agreement:

•: distributions of available cash from operating surplus on each common unit and subordinated unit equaled or exceeded $3.00 for each of the two consecutive, non-overlapping four-quarter periods immediately preceding that date;
•: the adjusted operating surplus generated during each of the two consecutive, non-overlapping four-quarter periods immediately preceding that date equaled or exceeded the sum of a distribution of $3.00 on all of the outstanding common units and subordinated units during those periods on a fully diluted basis and the related distribution on the 2% general partner interest during those periods; and
•: there are no arrearages in payment of the minimum quarterly distribution on the common units.

This additional early conversion is a one time occurrence.

There are currently 600,000 subordinated units outstanding. Generally, the earliest possible date by which all subordinated units may be converted into common units is June 30, 2007.

This additional early conversion is a one time occurrence.

Generally, the earliest possible date by which all subordinated units may be converted into common units is June 30, 2007.

Definition of Adjusted Operating Surplus. We define adjusted operating surplus in the glossary and for any period it generally means:

•: operating surplus generated during that period; less
•: any net increase in working capital borrowings during that period; less
•: any net reduction in cash reserves for operating expenditures during that period not relating to an operating expenditure made during that period; plus
•: any net decrease in working capital borrowings during that period; and plus
•: any net increase in cash reserves for operating expenditures during that period required by any debt instrument for the repayment of principal, interest or premium.

Adjusted operating surplus is intended to reflect the cash generated from operations during a particular period and therefore excludes net increases in working capital borrowings and net drawdowns of reserves of cash generated in prior periods.

Effect of Expiration of the Subordination Period. Upon expiration of the subordination period, each outstanding subordinated unit will convert into one common unit and will then participate pro rata with the other common units in distributions of available cash. In addition, if the unitholders remove our general partner other than for cause and units held by our general partner and its affiliates are not voted in favor of such removal, the subordination period will end, any then-existing arrearages on the common units will terminate and each subordinated unit will immediately convert into one common unit.

Distributions of Available Cash from Operating Surplus During the Subordination Period

We will make distributions of available cash from operating surplus for any quarter during the subordination period in the following manner:

•: First, 98% to the common unitholders, pro rata, and 2% to our general partner until we distribute for each outstanding common unit an amount equal to the minimum quarterly distribution for that quarter;
•: Second, 98% to the common unitholders, pro rata, and 2% to our general partner until we distribute for each outstanding common unit an amount equal to any arrearages in payment of the minimum quarterly distribution on the common units for any prior quarters during the subordination period;
•: Third, 98% to the subordinated unitholders, pro rata, and 2% to our general partner until we distribute for each subordinated unit an amount equal to the minimum quarterly distribution for that quarter; and
•: Thereafter, in the manner described in "—Incentive Distribution Rights" below.

Distributions of Available Cash from Operating Surplus After the Subordination Period

We will make distributions of available cash from operating surplus for any quarter after the subordination period in the following manner:

•: First, 98% to all unitholders, pro rata, and 2% to our general partner until we distribute for each outstanding unit an amount equal to the minimum quarterly distribution for that quarter; and
•: Thereafter, in the manner described in "—Incentive Distribution Rights" below.

Incentive Distribution Rights

Incentive distribution rights represent the right to receive an increasing percentage of quarterly distributions of available cash from operating surplus after the minimum quarterly distribution and the target distribution levels have been achieved. Our general partner currently holds the incentive distribution rights, but may transfer these rights separately from its general partner interest, subject to restrictions in the partnership agreement.

If for any quarter:

•: we have distributed available cash from operating surplus to the common and subordinated unitholders in an amount equal to the minimum quarterly distribution; and
•: we have distributed available cash from operating surplus on outstanding common units in an amount necessary to eliminate any cumulative arrearages in payment of the minimum quarterly distribution;

then, we will distribute any additional available cash from operating surplus for that quarter among the unitholders and our general partner in the following manner:

•: First, 98% to all unitholders, pro rata, and 2% to our general partner until each unit receives a total of $0.55 per unit for that quarter (the "first target distribution");
•: Second, 85% to all unitholders, pro rata, and 15% to our general partner, until each unitholder receives a total of $0.625 per unit for that quarter (the "second target distribution");
•: Third, 75% to all unitholders, pro rata, and 25% to our general partner, until each unitholder receives a total of $0.75 per unit for that quarter (the "third target distribution"); and

•: Thereafter, 50% to all unitholders, pro rata, and 50% to our general partner.

In each case, the amount of the target distribution set forth above is exclusive of any distributions to common unitholders to eliminate any cumulative arrearages in payment of the minimum quarterly distribution.

Target Amount of Quarterly Distribution

The following table illustrates the percentage allocations of the additional available cash from operating surplus between the unitholders and our general partner up to the various target distribution levels. The amounts set forth under "Marginal Percentage Interest in Distributions" are the percentage interests of the unitholders and our general partner in any available cash from operating surplus we distribute up to and including the corresponding amount in the column "Total Quarterly Distribution Target Amount," until available cash from operating surplus we distribute reaches the next target distribution level, if any. The percentage interests shown for the unitholders and our general partner for the minimum quarterly distribution are also applicable to quarterly distribution amounts that are less than the minimum quarterly distribution.

		Marginal Percentage Interest in Distributions
	Total Quarterly Distribution Target Amount	Unitholders		General Partner
Minimum Quarterly Distribution	$0.50	98	%	2	%
First Target Distribution	up to $0.55	98	%	2	%
Second Target Distribution	above $0.55 up to $0.625	85	%	15	%
Third Target Distribution	above $0.625 up to $0.75	75	%	25	%
Thereafter	above $0.75	50	%	50	%

Distributions from Capital Surplus

How Distributions from Capital Surplus Will Be Made. We will make distributions of available cash from capital surplus, if any, in the following manner:

•: First, 98% to all unitholders, pro rata, and 2% to our general partner, until we distribute for each common unit that was issued in the initial public offering, an amount of available cash from capital surplus equal to the initial public offering price;
•: Second, 98% to the common unitholders, pro rata, and 2% to our general partner, until we distribute for each common unit an amount of available cash from capital surplus equal to any unpaid arrearages in payment of the minimum quarterly distribution on the common units; and
•: Thereafter, we will make all distributions of available cash from capital surplus as if they were from operating surplus.

Effect of a Distribution from Capital Surplus. The partnership agreement treats a distribution of capital surplus as the repayment of the initial unit price from the initial public offering, which is a return of capital. The initial public offering price less any distributions of capital surplus per unit is referred to as the "unrecovered initial unit price." Each time a distribution of capital surplus is made, the minimum quarterly distribution and the target distribution levels will be reduced in the same proportion as the corresponding reduction in the unrecovered initial unit price. Because distributions of capital surplus will reduce the minimum quarterly distribution, after any of these distributions are made, it may be easier for our general partner to receive incentive distributions and for the subordinated units to convert into common units. However, any distribution of capital surplus before the unrecovered initial unit price is reduced to zero cannot be applied to the payment of the minimum quarterly distribution or any arrearages.

Once we distribute capital surplus on a unit in an amount equal to the initial unit price, we will reduce the minimum quarterly distribution and the target distribution levels to zero. We will then make all future distributions from operating surplus, with 50% being paid to the holders of units, 48% to the holders of the incentive distribution rights and 2% to our general partner.

Adjustment to the Minimum Quarterly Distribution and Target Distribution Levels

In addition to adjusting the minimum quarterly distribution and target distribution levels to reflect a distribution of capital surplus, if we combine our units into fewer units or subdivide our units into a greater number of units, we will proportionately adjust:

•: the minimum quarterly distribution;
•: target distribution levels;
•: unrecovered initial unit price;
•: the number of common units issuable during the subordination period without a unitholder vote; and
•: the number of common units into which a subordinated unit is convertible.

For example, if a two-for-one split of the common units should occur, the minimum quarterly distribution, the target distribution levels and the unrecovered initial unit price would each be reduced to 50% of its initial level. We will not make any adjustment by reason of the issuance of additional units for cash or property.

In addition, if legislation is enacted or if existing law is modified or interpreted in a manner that causes us to become taxable as a corporation or otherwise subject to taxation as an entity for federal, state or local income tax purposes, we will reduce the minimum quarterly distribution and the target distribution levels by multiplying the same by one minus the sum of the highest marginal federal corporate income tax rate that could apply and any increase in the effective overall state and local income tax rates. For example, if we became subject to a maximum marginal federal, and effective state and local income tax rate of 38%, then the minimum quarterly distribution and the target distributions levels would each be reduced to 62% of their previous levels.

Distributions of Cash upon Liquidation

General. If we dissolve in accordance with the partnership agreement, we will sell or otherwise dispose of our assets in a process called liquidation. We will first apply the proceeds of liquidation to the payment of our creditors. We will distribute any remaining proceeds to the unitholders and our general partner, in accordance with their capital account balances, as adjusted to reflect any gain or loss upon the sale or other disposition of our assets in liquidation.

The allocations of gain and loss upon liquidation are intended, to the extent possible, to entitle the holders of outstanding common units to a preference over the holders of outstanding subordinated units upon our liquidation, to the extent required to permit common unitholders to receive their unrecovered initial unit price plus the minimum quarterly distribution for the quarter during which liquidation occurs plus any unpaid arrearages in payment of the minimum quarterly distribution on the common units. However, there may not be sufficient gain upon our liquidation to enable the holders of common units to fully recover all of these amounts, even though there may be cash available for distribution to the holders of subordinated units. Any further net gain recognized upon liquidation will be allocated in a manner that takes into account the incentive distribution rights of our general partner.

Manner of Adjustments for Gain. The manner of the adjustment for gain is set forth in the partnership agreement. If our liquidation occurs before the end of the subordination period, we will allocate any gain to the partners in the following manner:

•

First, to our general partner and the holders of units who have negative balances in their capital accounts to the extent of and in proportion to those negative balances;

•

Second, 98% to the common unitholders, pro rata, and 2% to our general partner until the capital account for each common unit is equal to the sum of:

(1): the unrecovered initial unit price for that common unit; plus
(2): the amount of the minimum quarterly distribution for the quarter during which our liquidation occurs; and plus
(3): any unpaid arrearages in payment of the minimum quarterly distribution;

•

Third, 98% to the subordinated unitholders, pro rata, and 2% to our general partner until the capital account for each subordinated unit is equal to the sum of:

(1): the unrecovered initial unit price on that subordinated unit; and
(2): the amount of the minimum quarterly distribution for the quarter during which our liquidation occurs;

•

Fourth, 98% to all unitholders, pro rata, and 2% to our general partner, until we allocate under this paragraph an amount per unit equal to:

(1): the sum of the excess of the first target distribution per unit over the minimum quarterly distribution per unit for each quarter of our existence; less
(2): the cumulative amount per unit of any distributions of available cash from operating surplus in excess of the minimum quarterly distribution per unit that we distributed 98% to the unitholders, pro rata, and 2% to our general partner, for each quarter of our existence;

•

Fifth, 85% to all unitholders, pro rata, and 15% to our general partner, until we allocate under this paragraph an amount per unit equal to:

(1): the sum of the excess of the second target distribution per unit over the first target distribution per unit for each quarter of our existence; less
(2): the cumulative amount per unit of any distributions of available cash from operating surplus in excess of the first target distribution per unit that we distributed 85% to the unitholders, pro rata, and 15% to our general partner for each quarter of our existence;

•

Sixth, 75% to all unitholders, pro rata, and 25% to our general partner, until we allocate under this paragraph an amount per unit equal to:

(1): the sum of the excess of the third target distribution per unit over the second target distribution per unit for each quarter of our existence; less
(2): the cumulative amount per unit of any distributions of available cash from operating surplus in excess of the second target distribution per unit that we distributed 75% to the unitholders, pro rata, and 25% to our general partner for each quarter of our existence;

•

Thereafter, 50% to all unitholders, pro rata, and 50% to our general partner.

If the liquidation occurs after the end of the subordination period, the distinction between common units and subordinated units will disappear, so that clause (3) of the second priority above and all of the third priority above will no longer be applicable.

Manner of Adjustments for Losses. Upon our liquidation, we will generally allocate any loss to our general partner and the unitholders in the following manner:

•: First, 98% to holders of subordinated units in proportion to the positive balances in their capital accounts and 2% to our general partner until the capital accounts of the subordinated unitholders have been reduced to zero;
•: Second, 98% to the holders of common units in proportion to the positive balances in their capital accounts and 2% to our general partner until the capital accounts of the common unitholders have been reduced to zero; and
•: Thereafter, 100% to our general partner.

If the liquidation occurs after the end of the subordination period, the distinction between common units and subordinated units will disappear, so that all of the first priority above will no longer be applicable.

Adjustments to Capital Accounts. We will make adjustments to capital accounts upon the issuance of additional units. In doing so, we will allocate any unrealized and, for tax purposes, unrecognized gain or loss resulting from the adjustments to the unitholders and our general partner in the same manner as we allocate gain or loss upon liquidation. In the event that we make positive adjustments to the capital accounts upon the issuance of additional units, we will allocate any later negative adjustments to the capital accounts resulting from the issuance of additional units or upon our liquidation in a manner which results, to the extent possible, in our general partner's capital account balances equaling the amount which they would have been if no earlier positive adjustments to the capital accounts had been made.

SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

The following table sets forth the beneficial ownership of units as of July 12, 2006, held by beneficial owners of 5% or more of the units; by directors of our general partner; by each named executive officer listed in the summary compensation table incorporated by reference in this prospectus; and by all directors and officers of our general partner as a group.

Name of Beneficial Owner	Common Units Beneficially Owned(1)	Percentage of Common Units Beneficially Owned		Subordinated Units Beneficially Owned	Percentage of Subordinated Units Beneficially Owned		Percentage of Total Units Beneficially Owned
MarkWest Energy GP, L.L.C(2)	—	—		—	—		—
MarkWest Hydrocarbon, Inc.(3)	836,162	5.8	%	1,633,334	90.7	%	15.3	%
John M. Fox(4)	880,893	6.1	%	1,637,384	91.0	%	15.6	%
Kayne Anderson Capital Advisors, L.P(5) 1800 Avenue of the Stars, Second Floor Los Angeles, CA 90067	1,670,380	11.6	%	—	—		10.3	%
Richard A. Kayne(5) 1800 Avenue of the Stars, Second Floor Los Angeles, CA 90067	1,670,380	11.6	%	—	—		10.3	%
Tortoise Capital Advisors L.L.C.(6) 10801 Mastin Boulevard, Suite 222 Overland Park, KS 66210	966,704	6.7	%	—	—		6.0	%
Tortoise Energy Infrastructure Corporation(6) 10801 Mastin Boulevard, Suite 222 Overland Park, KS 66210	805,810	5.6	%	—	—		5.0	%
Tortoise MWEP, L.P.(7) 10801 Mastin Boulevard, Suite 222 Overland Park, KS 66210	—	—		166,666	9.3	%	1.0	%
Frank M. Semple(2)	13,734	*		—	*		—
James G. Ivey(2)	3,151	*		—	—		*
Randy S. Nickerson(2)	12,495	*		—	*		—
John C. Mollenkopf(2)	7,936	*		—	*		—
David L. Young(2)	136	*		—	—		*
Keith E. Bailey(2)	10,700	*		—	—		*
Donald C. Heppermann(2)	11,667	*		—	*		*
William A. Kellstrom(2)	4,042	*		—	—		*
William P. Nicoletti(2)	3,542	*		—	—		*
Charles K. Dempster(2)	1,042	*		—	—		*
All directors and executive officers as a group (17 persons)	951,927	6.6	%	1,637,384	91.0	%	16.0	%

*: Less than 1%
(1): Beneficial ownership for the purposes of the foregoing table is defined by Rule 13d-3 under the Securities Exchange Act of 1934. Under that rule, a person is generally considered to be the beneficial owner of a security if he has or shares the power to vote or direct the voting thereof ("Voting Power") or to dispose or direct the disposition thereof ("Investment Power") or has the right to acquire either of those powers within sixty (60) days.
(2): The address of each of these entities and individuals is 155 Inverness Drive West, Suite 200, Englewood, Colorado 80112.

(3): Includes securities owned directly and indirectly through subsidiaries.
(4): Includes 1,633,334 subordinated units owned by MarkWest Hydrocarbon and its subsidiaries, and approximately 4,050 subordinated units owned by Tortoise MWEP, L.P. in which Mr. Fox owns an equity interest. As of March 31, 2006, Mr. Fox beneficially owned approximately 43% of the voting securities of MarkWest Hydrocarbon. Mr. Fox currently serves as MarkWest Hydrocarbon's Chairman of the Board. Mr. Fox resigned as President of MarkWest Hydrocarbon effective November 1, 2003, and as Chief Executive Officer effective January 1, 2004. As a result, Mr. Fox may be deemed to be the beneficial owner of the common units and subordinated units owned by MarkWest Hydrocarbon.
(5): Information is based solely on a Schedule 13G filed with the Securities and Exchange Commission by Kayne Anderson Capital Advisors, L.P. and Richard A. Kayne, on February 9, 2006, with respect to units held as of December 31, 2005. The Schedule 13G indicates that Kayne Anderson Capital Advisors, L.P. and Richard A. Kayne have shared voting power and dispositive power with respect to 1,670,380 units. The reported units are owned by investment accounts managed, with discretion to purchase or sell securities, by Kayne Anderson Capital Advisors, L.P., as a registered investment advisor. Kayne Anderson Capital Advisors, L.P. is the general partner of the limited partnerships and investment adviser to the other accounts. Richard A. Kayne is the controlling shareholder of the corporate owner of Kayne Anderson Investment Management, Inc., the general partner of Kayne Anderson Capital Advisors, L.P. Mr. Kayne is also a limited partner of each of the limited partnerships and a shareholder of the registered investment company. Kayne Anderson Capital Advisors, L.P. disclaims beneficial ownership of the units reported, except those units attributable to it by virtue of its general partner interests in the limited partnerships. Mr. Kayne disclaims beneficial ownership of the units reported, except those units held by him or attributable to him by virtue of his limited partnership interests in the limited partnerships, his indirect interest in the interest of Kayne Anderson Capital Advisors, L.P. in the limited partnership, and his ownership of common units of the registered investment company.
(6): Information is based solely on a Schedule 13G filed with the Securities and Exchange Commission by Tortoise Capital Advisors, L.L.C. ("TCA") and Tortoise Energy Infrastructure Corporation ("TYG"), a closed-end investment company on February 10, 2006. The Schedule 13G indicates that TCA acts as an investment advisor to TYG and that TCA, and by virtue of the Investment Advisory Agreement with TYG, has all investment and voting power over securities owned of record by TYG. Despite its delegation of investment and voting power to TCA, however, TYG may be deemed to be the beneficial owner under Rule 13d-3 of the Securities and Exchange Act of 1940, of the securities it owns of record because it has the right to acquire investment and voting power through termination of the Investment Advisory Agreement. Thus, TCA and TYG have reported that they share voting power and dispositive power over the securities owned of record by TYG. TCA also acts as an investment advisor to certain managed accounts. Under contractual agreements with individual account holders, TCA, with respect to the securities held in the managed accounts, shares investment and voting power with certain account holders, and has no voting power but shares investment power with certain other account holders. TCA may be deemed the beneficial owner under Rule 13d-3 of the Exchange Act of the securities covered by the Schedule 13G. None of the securities are owned of record by TCA, and TCA disclaims any beneficial interest in such shares.
(7): Tortoise Capital General Partner, L.C., the general partner of Tortoise MWEP, L.P., is a limited liability company whose sole member is TCA. Tortoise Capital General Partner, L.C. exercises voting and dispositive power with respect to the subordinated units held by Tortoise MWEP, L.P.

The following table sets forth the beneficial ownership of our general partner as of June 15, 2006, held by MarkWest Hydrocarbon, the directors of our general partner, each named executive officer and by all directors and officers of our general partner as a group.

Name of Beneficial Owner	Percentage of Limited Liability Company Interests Owned
MarkWest Hydrocarbon, Inc.	89.2	%
John M. Fox(1)	90.8
Frank M. Semple	2.0
James G. Ivey	0.5
Randy S. Nickerson	1.6
John C. Mollenkopf	1.6
David L. Young	—
Keith E. Bailey	—
Donald C. Heppermann	1.0
William A. Kellstrom	—
William P. Nicoletti	—
Charles K. Dempster	—
All directors and executive officers as a group (17 persons)	98.0

(1): Includes a 1.6% ownership interest held directly by Mr. Fox and an 89.2% ownership interest held by MarkWest Hydrocarbon. As of December 31, 2005, Mr. Fox beneficially owned approximately 43% of the voting securities of MarkWest Hydrocarbon. Mr. Fox currently serves as MarkWest Hydrocarbon's Chairman of the Board. Mr. Fox resigned as President of MarkWest Hydrocarbon effective November 1, 2003, and as Chief Executive Officer effective January 1, 2004. As a result, Mr. Fox may be deemed to be the beneficial owner of the ownership interests owned by MarkWest Hydrocarbon.

SELLING UNITHOLDERS

This prospectus covers the offering for resale of up to 3,820,097 common units by selling unitholders. Unless otherwise indicated, each of the selling unitholders acquired its common units in connection with our June 2003, July 2004, November 2005 or December 2005 private placements. We and the selling unitholders entered into various registration rights agreements in connection with those private placements. We are registering the common units described below pursuant to such registration rights agreements.

No offer or sale may be made by a unitholder unless that unitholder is listed in the table below. The selling unitholders may sell all, some or none of the common units covered by this prospectus. Please read "Plan of Distribuiton." We will bear all costs, fees and expenses incurred in connection with the registration of the common units offered by this prospectus. Brokerage commissions and similar selling expenses, if any, attributable to the sale of common units will be borne by the selling unitholders.

No such sales may occur unless the selling unitholder has notified us of his or her intention to sell our common units and this prospectus has been declared effective by the SEC, and remains effective at the time such selling unitholder offers or sells such common units. We are required to update this prospectus to reflect material developments in our business, financial position and results of operations.

The following table sets forth, the name of each selling unitholder, the amount of common units beneficially owned and the percentage of common units outstanding owned by each selling unitholder prior to the offering, the number of common units being offered for each selling unitholder's account, and the amount to be owned and the percentage of common units outstanding owned by each selling unitholder following the completion of the offering (assuming each selling unitholder sells all of the common units covered by this prospectus). The percentages of common units outstanding have been calculated based on 14,379,214 common units outstanding as of July 12, 2006. Unless otherwise indicated, the selling unitholders have held no position or office or had any other material relationship with us or any of our affiliates or predecessors, other than as a unitholder, during the past three years.

We have prepared the table and the related notes based on information supplied to us by the selling unitholders. We have not sought to verify such information. Additionally, some or all of the selling unitholders may have sold or transferred some or all of the units listed below in exempt or non-exempt transactions since the date on which the information was provided to us. Other information about the selling unitholders may change over time.

	Common Units Beneficially Owned Prior to this Offering			Common Units Beneficially Owned Following this Offering
Name of Selling Unitholder	Number	Percentage Owned	Units Offered in this Offering	Number	Percentage Owned
Kayne Anderson Energy Fund I, L.P.(1)	433,200	3.0	433,200	0	0
Kayne Anderson Capital Income Partners (QP), L.P.(1)	17,000	*	17,000	0	0
Kayne Anderson MLP Fund, L.P.(1)	280,587	2.0	72,000	208,587	1.5
Kayne Anderson Capital Income Fund, Ltd.(1)	14,000	*	14,000	0	0
Kayne Anderson Capital Income Partners, L.P.(1)	11,100	*	6,000	5,100	*
HFR RV Performance Master Trust(2)	20,300	*	5,800	14,500	*
Kayne Anderson MLP Investment Company(3)	871,680	6.1	678,580	193,100	1.3
Tortoise Energy Infrastructure Corporation(4)	805,810	5.6	579,710	226,100	1.6

Energy Income and Growth Fund(5)	230,178	1.6	144,928	85,250	*
Fiduciary/Claymore MLP Opportunity Fund(6)	299,347	2.1	113,097	186,250	1.3
Alerian Capital Partners LP(7)	80,935	*	63,335	17,600	*
Structured Finance Americas, LLC(8)	395,600	2.8	340,000	55,600	*
Credit Suisse Management LLC(9)	60,000	*	60,000	0	0
RCH Energy MLP Fund, L.P.(10)	184,182	1.3	110,982	73,200	*
RCH Energy MLP Fund-A, L.P.(10)	2,115	*	2,115	0	0
RCH Energy Opportunity Fund , L.P.(11)	341,137	2.4	341,137	0	0
Fort Mason Partners, L.P.(12)	9,800	*	9,800	0	0
Fort Mason Master, L.P.(12)	151,120	1.0	151,120	0	0
Swank MLP Convergence Fund, LP(13)	327,485	2.3	229,885	97,600	*
Eagle Income Appreciation Partners, L.P.(14)	121,766	*	68,966	52,800	*
Royal Bank of Canada(15)	73,877	*	49,762	24,115	*
Delmar Equity Partners, L.P.(16)	35,947	*	19,062	16,885	*
Jeane Marie Swalm(17)	4,547	*	4,547	0	0
Robert A. Tucci and Cynthia L. Tucci TEN(18)	3,731	*	3,731	0	0
Evan D. Jennings, II(19)	16,000	*	16,000	0	0
Lewis E. Nerman Revocable Trust Dtd 10/19/89, as amended, Lewis E. Nerman, Trustee(17)	28,757	*	15,249	13,508	*
Richard B. Klein Revocable Trust dated 6/08/1993, Richard B. Klein and Lynn M. Klein, Trustees(17)	6,031	*	6,031	0	0
Jerome S. Nerman Trust UA dated 11/08/88, Jerome S. Nerman, Trustee(17)	7,624	*	7,624	0	0
Gary M. Truitt and Kay Lee Truitt JT TEN(18)	16,067	*	7,624	8,443	*
Laura Marcia Wolff Greenbaum Trust Dated 9/20/78, Laura W. Greenbaum Trustee(17)	3,960	*	2,268	1,692	*
Peter P. Dreher and Nancy D. Dreher JT TEN(17)	1,000	*	1,000	0	0
Thomas M. Cray Revocable Trust dated 10/9/2000, Thomas M. Cray Trustee(17)	1,906	*	1,906	0	0
Robyn R. Schneider Revocable Trust dated 3/25/99, Robyn R. Schneider Trustee(20)	1,000	*	1,000	0	0
Sandra H. Fried Trust U/I/T Dated July 25, 1996, as amended and restated July 25, 2002, Sandra H. Fried Trustee(17)	1,906	*	1,906	0	0
Stuart M. Bauman Revocable Trust U/A Dtd. 1/28/93, Stuart M. Bauman, Trustee(19)	1,143	*	1,143	0	0
Cara Zanotti Newell(21)	953	*	953	0	0
Charles M. Newell(19)	953	*	953	0	0

James Thomas and Cynthia F. Burcham; JTWROS(19)		7,624	*	7,624	0	0
H. Kevin Birzer(22)		3,924	*	3,813	111	*
KC Partners in Growth(23)		3,812	*	3,812	0	0
IMREX Partners, L.P.(25)		762	*	762	0	0
Ellen E. Schulte(22)		2,978	*	1,965	1,013	*
Douglas E. Campbell(24)		1,906	*	1,906	0	0
Donald W. Trotter(24)		1,143	*	1,143	0	0
Larry H. Powell(24)		1,064	*	953	111	*
Terry Matlack(24)		2,787	*	1,583	1,204	*
Steven W. Satkamp(24)		7,624	*	7,624	0	0
Gregory S. and Elizabeth B. Maday, JWTROS(24)		20,180	*	6,672	13,508	*
Zachary A. Hamel(24)		1,486	*	953	533	*
Kenneth P. Malvey(24)		1,521	*	953	568	*
SLK Family Trust, Sharon L. Kessler, Trustee(24)		750	*	750	0	0
Frank M. Semple(26)		13,734	*	5,000	8,734	*
John M. Fox(27)		44,731	*	4,626	40,105	*
John C. Mollenkopf(28)		7,936	*	4,626	3,310	*
Randy S. Nickerson(29)		12,495	*	4,626	7,869	*
Arthur J. Denney(30)		8,876	*	4,626	4,250	*
Donald C. Heppermann(31)		11,667	*	4,000	7,667	*
Andrew Schroeder(32)		2,203	*	1,500	703	*
Ted S. Smith(33)		2,720	*	1,500	1,220	*
Tortoise MWEP, L.P.(34)		166,666	1.1	166,666	0	0
	Total:	5,191,323	36.1	3,820,097	1,370,486	9.5

*: Less than 1%.
(1): As of March 27, 2006. Richard A. Kayne, in his capacity as the controlling stockholder of Kayne Anderson Investment Management, Inc. ("Kayne Management"), the ultimate parent of the selling unitholder, may be deemed to have voting and dispositive power with respect to the units held by the selling unitholder. KA Associates, Inc., an affiliate of the selling unitholder, is a broker-dealer registered pursuant to Section 15(b) of the Securities and Exchange Act of 1934, as amended (the "Exchange Act") and is a member of the National Association of Securities Dealers, Inc., or NASD. The selling unitholder has represented that (i) it purchased the securities for the selling unitholder's own account, not as a nominee or agent, in the ordinary course of business and with no intention of selling or otherwise distributing such securities in any transaction in violation of securities laws and (ii) at the time of purchase, the selling unitholder did not have any agreement or understanding, direct or indirect, with any other person to sell or otherwise distribute the purchased securities.
(2): As of March 27, 2006. Pursuant to investment advisory agreements entered into with the selling unitholder, Kayne Anderson Capital Advisors, L.P. ("Kayne Advisors") has voting and dispositive power with respect to the units held by the selling unitholder. Richard A. Kayne, in his capacity as the controlling stockholder of Kayne Management, the general partner of Kayne Advisors, may be

deemed to have voting and dispositive power with respect to the units held by the selling unitholder.

(3): As of March 28, 2006. The selling unitholder is a publicly-held closed-end fund. Pursuant to investment advisory agreements entered into with the selling unitholder, Kayne Advisors has voting and dispositive power with respect to the units held by the selling unitholder. Richard A. Kayne, in his capacity as the controlling stockholder of Kayne Management, the general partner of Kayne Advisors, may be deemed to have voting and dispositive power over the units held by the selling unitholder.
(4): As of March 27, 2006. Tortoise Capital Advisors, L.L.C. ("TCA") serves as the investment advisor to the selling unitholder. Pursuant to a Investment Advisory Agreement entered into with the selling unitholder, TCA holds voting and dispositive power with respect to the units held by the selling unitholder. The investment committee of TCA is responsible for the investment management of the selling unitholder's portfolio. The investment committee is comprised of H. Kevin Birzer, Zachary A. Hamel, Kenneth P. Malvey, Terry C. Matlack and David J. Schulte.
(5): As of March 24, 2006. Does not include 431,957 common units controlled and managed by Fiduciary Asset Management, LLC ("FAMCO"), an affiliate of the selling unitholder. Pursuant to investment advisory agreements entered into with the selling unitholder, FAMCO holds voting and dispositive power with respect to the units held by the selling unitholder. The investment committee of FAMCO is responsible for the investment management of the selling unitholder's portfolio. The investment committee of FAMCO is comprised of Charles D. Walbrandt, Wiley D. Angell, Joseph E. Gallager, James J. Cunnane, Jr., Mohammed Riad, Thomas L. Engle, Timothy Swanson, Quinn T. Kiley, Katherine K. Dienner and William N. Adams. First Trust Portfolios L.P., an affiliate of the selling unitholder, is a broker-dealer registered pursuant to Section 15(b) of the Exchange Act and a member of the NASD. The selling unitholder has represented that (i) it purchased the securities for the selling unitholder's own account, not as a nominee or agent, in the ordinary course of business and with no intention of selling or otherwise distributing such securities in any transaction in violation of securities laws and (ii) at the time of purchase, the selling unitholder did not have any agreement or understanding, direct or indirect, with any other person to sell or otherwise distribute the purchased securities.
(6): As of March 24, 2006. Does not include 463,788 common units controlled and managed by Fiduciary Asset Management, LLC ("FAMCO"), an affiliate of the selling unitholder. Pursuant to investment advisory agreements entered into with the selling unitholder, FAMCO holds voting and dispositive power with respect to the units held by the selling unitholder. The investment committee of FAMCO is responsible for the investment management of the selling unitholder's portfolio. The investment committee of FAMCO is comprised of Charles D. Walbrandt, Wiley D. Angell, Joseph E. Gallager, James J. Cunnane, Jr., Mohammed Riad, Thomas L. Engle, Timothy Swanson, Quinn T. Kiley, Katherine K. Dienner and William N. Adams. Claymore Securities, Inc., an affiliate of the selling unitholder, is a broker-dealer registered pursuant to Section 15(b) of the Exchange Act and a member of the NASD. The selling unitholder has represented that (i) it purchased the securities for the selling unitholder's own account, not as a nominee or agent, in the ordinary course of business and with no intention of selling or otherwise distributing such securities in any transaction in violation of securities laws and (ii) at the time of purchase, the selling unitholder did not have any agreement or understanding, direct or indirect, with any other person to sell or otherwise distribute the purchased securities.
(7): As of March 23, 2006. Alerian Capital Management LLC ("Alerian Management") holds voting and dispositive power with respect to the units held by the selling unitholder Gabriel Hammond, in his capacity controlling shareholder of Alerian Management, may be deemed to have voting and dispositive power with respect to the units held by the selling unitholder.

(8): As of March 27, 2006. Deutsche Bank AG, a German banking corporation, holds voting and dispositive power with respect to the shares held by the selling unitholder. Deutsche Bank Securities Inc., an affiliate of the selling unitholder, is a broker-dealer registered pursuant to Section 15(b) of the Exchange Act and a member of the NASD. The selling unitholder has represented that (i) it purchased the securities for the selling unitholder's own account, not as a nominee or agent, in the ordinary course of business and with no intention of selling or otherwise distributing such securities in any transaction in violation of securities laws and (ii) at the time of purchase, the selling unitholder did not have any agreement or understanding, direct or indirect, with any other person to sell or otherwise distribute the purchased securities.
(9): As of March 27, 2006. Credit Suisse (USA) Inc., a Delaware corporation ("CSFB-USA"), holds voting and dispositive power with respect to the shares held by the selling unitholder. Credit Suisse, a Swiss bank (the "Bank"), owns the majority of the voting stock of Credit Suisse Holdings (USA), Inc., a Delaware corporation, which in turn owns all of the voting stock of CSFB-USA. The ultimate parent company of the Bank is Credit Suisse Group ("CSG"). CSG disclaims beneficial ownership of the reported common stock that is beneficially owned by its direct and indirect subsidiaries. Credit Suisse Securities (USA) LLC, an affiliate of the selling unitholder, is a broker-dealer registered pursuant to Section 15(b) of the Exchange Act and a member of the NASD. The selling unitholder has represented that (i) it purchased the securities for the selling unitholder's own account, not as a nominee or agent, in the ordinary course of business and with no intention of selling or otherwise distributing such securities in any transaction in violation of securities laws and (ii) at the time of purchase, the selling unitholder did not have any agreement or understanding, direct or indirect, with any other person to sell or otherwise distribute the purchased securities.
(10): As of March 28, 2006. The general partner of the selling unitholder is RCH Energy MLP Fund GP, L.P. ("RCH MLP"). Robert J. Raymond, as member of RR Advisors, LLC, the general partner of RCH MLP, exercises voting and dispositive power with respect to the units held by the selling unitholder.
(11): As of March 28, 2006. The general partner of the selling unitholder is RCH Energy Opportunity Fund I GP, L.P. ("RCH Opportunity"). Robert J. Raymond, as member of RR Advisors, LLC, the general partner of RCH Opportunity, exercises voting and dispositive power with respect to the units held by the selling unitholder.
(12): As of March 23, 2006. Fort Mason Capital, LLC serves as the general partner and/or investment advisor to the selling unitholder and, in such capacity, exercises sole voting and investment authority with respect to such units, Daniel German serves as the sole managing member of Fort Mason Capital, LLC. Fort Mason Capital, LLC and Mr. German each disclaim beneficial ownership of such units, except to the extent of its or his pecuniary interest therein, if any.
(13): As of March 22, 2006. Swank Energy Income Advisors, L.P. ("Swank Advisors") holds voting and dispositive power with respect to the units held by the selling unitholder. Jerry V. Swank, as controlling shareholder of Swank Group, LLC, the general partner of Swank Advisors, may be deemed to have voting and dispositive power with respect to the units held by the selling unitholder.
(14): As of March 22, 2006. Eagle Global Advisors, LLC holds voting and dispositive power with respect to the units held by the selling unitholder. The shareholders of Eagle Global Advisors, LLC are Edward R. Allen, III, Thomas N. Hunt, III, Steven Russo, John Gualy and Malcom Day.
(15): As of April 4, 2006. The selling unitholder, a publicly-traded banking corporation, exercises voting and dispositive power with respect to the units held by the selling unitholder. The selling unitholder serves as administrative agent and lead arranger for our credit facility and MarkWest

Hydrocarbon's revolving credit facility and receives customary fees for such services in the ordinary course of business. RBC Capital Markets Corporation ("RBC Capital Markets"), an affiliate of the selling unitholder, in the ordinary course of business, provides us with financial advisory and investment banking services in exchange for customary fees and expenses. In connection with these services, RBC Capital Markets served as an underwriter in our January 2004 and September 2004 common unit offerings and as an initial purchaser in our November 2004 private placement of senior notes. RBC Capital Markets is a broker-dealer registered pursuant to Section 15(b) of the Exchange Act and a member of the NASD. The selling unitholder has represented that (i) it purchased the securities for the selling unitholder's own account, not as a nominee or agent, in the ordinary course of business and with no intention of selling or otherwise distributing such securities in any transaction in violation of securities laws and (ii) at the time of purchase, the selling unitholder did not have any agreement or understanding, direct or indirect, with any other person to sell or otherwise distribute the purchased securities.

(16): As of March 28, 2006. Mark R. Truitt and Linda A Truitt exercise joint voting and dispositive power of the units held by the selling unitholder.
(17): As of March 29, 2006.
(18): As of March 28, 2006.
(19): As of March 30, 2006.
(20): As of March 25, 2006.
(21): As of March 27, 2006.
(22): As of April 4, 2006.
(23): As of March 31, 2006. Kenneth P. Malvey exercises voting and dispositive power with respect to the shares held by the selling unitholder.
(24): As of March 31, 2006.
(25): As of March 31, 2006. Terry Matlack, Timothy Matlack, Rex Matlack and Cindy Matlack Manley exercise joint voting and dispositive power with respect to the units held by the selling unitholder.
(26): As of April 4, 2006. Mr. Semple serves as President and Chief Executive Officer of MarkWest Hydrocarbon and our general partner and as a director of our general partner. The common units being offered by Mr. Semple were acquired upon the conversion of subordinated units acquired pursuant to the MarkWest Hydrocarbon Participation Plan.
(27): As of April 4, 2006. Mr. Fox serves as Chairman of the Board of MarkWest Hydrocarbon and our general partner. The common units being offered by Mr. Fox were acquired upon the conversion of subordinated units acquired pursuant to the MarkWest Hydrocarbon Participation Plan.
(28): As of April 4, 2006. Mr. Mollenkopf serves as Senior Vice President, Southwest Business Unit, of MarkWest Hydrocarbon and our general partner. The common units being offered by Mr. Mollenkopf were acquired upon the conversion of subordinated units acquired pursuant to the MarkWest Hydrocarbon Participation Plan.
(29): As of April 4, 2006. Mr. Nickerson serves as Senior Vice President, Corporate Development, of MarkWest Hydrocarbon and our general partner. The common units being offered by Mr. Nickerson were acquired upon the conversion of subordinated units acquired pursuant to the MarkWest Hydrocarbon Participation Plan.

(30): As of April 4, 2006. Mr. Denney served as Senior Vice President and Chief Operating Officer of MarkWest Hydrocarbon and our general partner until February 2004. The common units being offered by Mr. Denney were acquired upon the conversion of subordinated units acquired pursuant to the MarkWest Hydrocarbon Participation Plan.
(31): As of April 4, 2006. Mr. Heppermann served as Executive Vice President, Chief Financial Officer and Secretary of MarkWest Hydrocarbon and our general partner until his retirement in March 2004. The common units being offered by Mr. Hepperman were acquired upon the conversion of subordinated units acquired pursuant to the MarkWest Hydrocarbon Participation Plan.
(32): As of April 4, 2006. Mr. Schroeder serves as Vice President, Finance and Treasurer of MarkWest Hydrocarbon and our general partner. The common units being offered by Mr. Schroeder were acquired upon the conversion of subordinated units acquired pursuant to the MarkWest Hydrocarbon Participation Plan.
(33): As of April 4, 2006. Mr. Smith previously served as Controller and Chief Accounting Officer of MarkWest Hydrocarbon and our general partner. The common units being offered by Mr. Smith were acquired upon the conversion of subordinated units acquired pursuant to the MarkWest Hydrocarbon Participation Plan.
(34): As of March 27, 2006. The common units attributed to the selling unitholder represent common units underlying subordinated units held by the selling unitholder. The subordinated units held by the selling unitholder will convert into common units on a one-for-one basis as described in "Cash Distribution Policy—Subordination Period." The subordinated units were acquired by the selling unitholder in connection with the private placement of our subordinated units by Markwest Hydrocarbon in November 2002. The common units underlying the subordinated units held by the selling unitholder are being registered pursuant to a registration rights agreement among us, MarkWest Hydrocarbon and the selling unitholder. Tortoise Capital General Partner, L.C., the general partner of the selling unitholder, is a limited liability company whose sole member is Tortoise Capital Advisors, L.L.C. The investment committee of Tortoise Capital Advisors L.L.C. is comprised of H. Kevin Birzer, Zachary A. Hamel, Kenneth P. Malvey, Terry C. Matlack and David J. Schulte. Tortoise Capital General Partner, L.C. exercises voting and dispositive power with respect to the subordinated units held by the selling unitholder.

CONFLICTS OF INTEREST AND FIDUCIARY DUTIES

Conflicts of interest exist and may arise in the future as a result of the relationships between our general partner and its affiliates (including MarkWest Hydrocarbon), on the one hand, and us, and our limited partners, on the other hand. The directors and officers of our general partner have fiduciary duties to manage our general partner in a manner beneficial to its owners. At the same time, our general partner has a fiduciary duty to manage MarkWest Energy Partners, L.P., in a manner beneficial to us and our unitholders. In addition, officers of our general partner and officers and key employees of MarkWest Hydrocarbon also own 10.3% of the membership interests in our general partner, a significant equity stake in MarkWest Hydrocarbon and own approximately 1.0% of the limited partner interests in us.

The partnership agreement contains provisions that allow our general partner to take into account the interests of parties in addition to our interests when resolving conflicts of interest. In effect, these provisions limit our general partner's fiduciary duties to the unitholders. The partnership agreement also restricts the remedies available to unitholders for actions taken that, without those limitations, might constitute breaches of fiduciary duty. Whenever a conflict arises between our general partner or its affiliates, on the one hand, and us. or any other partner, on the other, our general partner will resolve that conflict. At the request of our general partner, a conflicts committee of the board of directors of our general partner will review conflicts of interest. Our general partner will not be in breach of its obligations under the partnership agreement or its duties to us or the unitholders if the resolution of the conflict is considered fair and reasonable to us. Any resolution is considered fair and reasonable to us if that resolution is:

•: approved by the conflicts committee, although no party is obligated to seek the approval of the Conflicts Committee and our general partner may adopt a resolution or course of action that has not received approval;
•: on terms no less favorable to us than those generally being provided to or available from unrelated third parties; or
•: fair to us, taking into account the totality of the relationships between the parties involved, including other transactions that may be particularly favorable or advantageous to us.

Unless the resolution is specifically provided for in the partnership agreement, when resolving a conflict, our general partner may consider:

•: the relative interests of the parties involved in the conflict or affected by the action;
•: any customary or accepted industry practices or historical dealings with a particular person or entity; and
•: generally accepted accounting practices or principles and other factors it considers relevant, if applicable.

Conflicts of interest could arise in the situations described below, among others:

Actions taken by our general partner may affect the amount of cash available for distribution to unitholders or accelerate the right to convert subordinated units.

The amount of cash that is available for distribution to unitholders is affected by decisions of our general partner regarding such matters as:

•: amount and timing of asset purchases and sales;
•: cash expenditures;
•: borrowings;

•: issuance of additional units; and
•: the creation, reduction or increase of reserves in any quarter.

In addition, borrowings by us and our affiliates do not constitute a breach of any duty owed by our general partner to our unitholders, including borrowings that have the purpose or effect of:

•: enabling our general partner or its affiliates to receive distributions on any subordinated units held by them or the incentive distribution rights; or
•: hastening the expiration of the subordination period.

For example, in the event we have not generated sufficient cash from our operations to pay the minimum quarterly distribution on our common units and our subordinated units, the partnership agreement permits us to borrow funds, which would enable us to make this distribution on all outstanding units. Please read "Cash Distribution Policy—Subordination Period."

The partnership agreement provides that we and our subsidiaries may borrow funds from our general partner and its affiliates. Our general partner and its affiliates may not borrow funds from us, the operating company or the subsidiaries.

We do not have any officers or employees and rely solely on officers of our general partner and employees of MarkWest Hydrocarbon and its affiliates.

We do not have any officers or employees and rely solely on officers and employees of MarkWest Hydrocarbon and its affiliates. MarkWest Hydrocarbon and its affiliates conduct businesses and activities of their own in which we have no economic interest. If these separate activities are significantly greater than our activities, there could be material competition for the time and effort of the officers and employees who provide services to our general partner. The officers of our general partner are not required to work full time on our affairs. These officers are required to devote significant time to the affairs of MarkWest Hydrocarbon or its affiliates and are compensated by them for the services rendered to them.

We reimburse our general partner, MarkWest Hydrocarbon and its affiliates for expenses.

We reimburse our general partner, MarkWest Hydrocarbon and its affiliates for costs incurred in managing and operating us, including costs incurred in rendering corporate staff and support services to us. The partnership agreement provides that our general partner determines the expenses that are allocable to us in any reasonable manner determined by our general partner in its sole discretion. See Item 13. "Certain Relationships and Related Transactions—Omnibus Agreement—Services" incorporated by reference to our Annual Report on Form 10-K for the year ended December 31, 2005, as amended.

Our general partner intends to limit its liability regarding our obligations.

Our general partner intends to limit its liability under contractual arrangements so that the other party has recourse only to our assets and not against our general partner or its assets or any affiliate of the general partner or its assets. The partnership agreement provides that any action taken by our general partner to limit its or our liability is not a breach of our general partner's fiduciary duties, even if we could have obtained terms that are more favorable without the limitation on liability.

Common unitholders have no right to enforce obligations of our general partner and its affiliates under agreements with us.

Any agreements between us, on the one hand, and our general partner and its affiliates, on the other, will not grant to the unitholders, separate and apart from us, the right to enforce the obligations of our general partner and its affiliates in our favor.

Contracts between us, on the one hand, and our general partner and its affiliates, on the other, will not be the result of arm's-length negotiations.

The partnership agreement allows our general partner to pay itself or its affiliates for any services rendered, provided these services are rendered on terms that are fair and reasonable to us. Our general partner may also enter into additional contractual arrangements with any of its affiliates on our behalf. Neither the partnership agreement nor any of the other agreements, contracts and arrangements between us and our general partner and its affiliates are or will be the result of arm's-length negotiations. However, all of these transactions are to be on terms that are fair and reasonable to us.

Our general partner and its affiliates have no obligation to permit us to use any facilities or assets of our general partner and its affiliates, except as may be provided in contracts entered into specifically dealing with that use. There is no obligation of our general partner and its affiliates to enter into any contracts of this kind.

Common units are subject to our general partner's limited call right.

Our general partner may exercise its right to call and purchase common units as provided in the partnership agreement or assign this right to one of its affiliates or to us. Our general partner may use its own discretion, free of fiduciary duty restrictions, in determining whether to exercise this right. As a result, a common unitholder may have his common units purchased from him at an undesirable time or price. Please read "The Partnership Agreement—Limited Call Right."

We may not choose to retain separate counsel for ourselves or for the holders of common units.

Attorneys, independent accountants and others who perform services for us have been retained by our general partner. Attorneys, independent accountants and others who perform services for us are selected by our general partner or the conflicts committee and may perform services for our general partner and its affiliates. We may retain separate counsel for ourselves or the holders of common units in the event of a conflict of interest between our general partner and its affiliates, on the one hand, and us or the holders of common units, on the other, depending on the nature of the conflict. We do not intend to do so in most cases.

Our general partner's affiliates may compete with us.

The partnership agreement provides that our general partner is restricted from engaging in any business activities other than those incidental to its ownership of interests in us. Except as provided in the partnership agreement and the omnibus agreement, affiliates of our general partner are not prohibited from engaging in other businesses or activities, including those that might be in direct competition with us.

Fiduciary duties owed to unitholders by our general partner are prescribed by law and the partnership agreement.

Our general partner is accountable to us and our unitholders as a fiduciary. The Delaware Revised Uniform Limited Partnership Act, which we refer to in this prospectus as the Delaware Act, provides

that Delaware limited partnerships may, in their partnership agreements, restrict or expand the fiduciary duties owed by our general partner to limited partners and us.

Our partnership agreement contains various provisions restricting the fiduciary duties that might otherwise be owed by our general partner. We have adopted these restrictions to allow our general partner to take into account the interests of other parties in addition to our interests when resolving conflicts of interest. We believe this is appropriate and necessary because our general partner's board of directors have fiduciary duties to manage our general partner in a manner beneficial both to its owners, MarkWest Hydrocarbon, as well as to you. Without these modifications, the general partner's ability to make decisions involving conflicts of interest would be restricted. The modifications to the fiduciary standards benefit the general partner by enabling it to take into consideration all parties involved in the proposed action, so long as the resolution is fair and reasonable to us as described above. These modifications also enable our general partner to attract and retain experienced and capable directors. These modifications represent a detriment to the common unitholders because they restrict the remedies available to unitholders for actions that, without those limitations, might constitute breaches of fiduciary duty, as described below. The following is a summary of the material restrictions of the fiduciary duties owed by our general partner to the limited partners:

State-law fiduciary duty standards		Fiduciary duties are generally considered to include an obligation to act with due care and loyalty. The duty of care, in the absence of a provision in a partnership agreement providing otherwise, would generally require a general partner to act for us in the same manner as a prudent person would act on his own behalf. The duty of loyalty, in the absence of a provision in a partnership agreement providing otherwise, would generally prohibit a general partner of a Delaware limited partnership from taking any action or engaging in any transaction where a conflict of interest is present.
Partnership agreement modified standards		Our partnership agreement contains provisions that waive or consent to conduct by our general partner and its affiliates that might otherwise raise issues as to compliance with fiduciary duties or applicable law. For example, our partnership agreement permits our general partner to make a number of decisions in its "sole discretion." This entitles our general partner to consider only the interests and factors that it desires and it has no duty or obligation to give any consideration to any interest of, or factors affecting, us, our affiliates or any limited partner. Other provisions of the partnership agreement provide that our general partner's actions must be made in its reasonable discretion. These standards reduce the obligations to which our general partner would otherwise be held. Our partnership agreement generally provides that affiliated transactions and resolutions of conflicts of interest not involving a required vote of unitholders must be "fair and reasonable" to us under the factors previously set forth. In determining whether a transaction or resolution is "fair and reasonable," our general partner may consider interests of all parties involved, including its own. Unless our general partner has acted in bad faith, the action taken by our general partner shall not constitute a breach of its fiduciary duty. These standards reduce the obligations to which our general partner would otherwise be held.

		In addition to the other more specific provisions limiting the obligations of our general partner, our partnership agreement further provides that our general partner and its officers and directors will not be liable for monetary damages to us, the limited partners or assignees for errors of judgment or for any acts or omissions if our general partner and those other persons acted in good faith.
Rights and Remedies of unitholders		The Delaware Act generally provides that a limited partner may institute legal action on behalf of us to recover damages from a third party where a general partner has refused to institute the action or where an effort to cause a general partner to do so is not likely to succeed. These actions could include actions against a general partner for breach of its fiduciary duties or of the partnership agreement. In addition, the statutory or case law of some jurisdictions may permit a limited partner to institute legal action on behalf of himself and all other similarly situated limited partners to recover damages from a general partner for violations of its fiduciary duties to the limited partners.

In order to become one of our limited partners, a common unitholder is required to agree to be bound by the provisions in the partnership agreement, including the provisions discussed above. This is in accordance with the policy of the Delaware Act favoring the principle of freedom of contract and the enforceability of partnership agreements. The failure of a limited partner or assignee to sign a partnership agreement does not render the partnership agreement unenforceable against that person.

We must indemnify our general partner and its officers, directors, employees, affiliates, partners, members, agents and trustees, to the fullest extent permitted by law, against liabilities, costs and expenses incurred by our general partner or these other persons. We must provide this indemnification if our general partner or these persons acted in good faith and in a manner they reasonably believed to be in, or (in the case of a person other than our general partner) not opposed to, our best interests. We also must provide this indemnification for criminal proceedings if our general partner or these other persons had no reasonable cause to believe their conduct was unlawful. Thus, our general partner could be indemnified for its negligent acts if it met these requirements concerning good faith and our best interests. To the extent that these provisions purport to include indemnification for liabilities arising under the Securities Act, in the opinion of the Securities and Exchange Commission, such indemnification is contrary to public policy and therefore unenforceable. If you have questions regarding the fiduciary duties of our general partner, you should consult with your own counsel. Please read "The Partnership Agreement—Indemnification."

THE PARTNERSHIP AGREEMENT

The following is a summary of the material provisions of our partnership agreement. Our partnership agreement and the limited liability company agreement governing our operating company are included as exhibits to the registration statement of which this prospectus constitutes a part. We will provide prospective investors with a copy of these agreements upon request at no charge.

We summarize the following provisions of the partnership agreement elsewhere in this prospectus:

•: with regard to distributions of available cash, please read "Cash Distribution Policy."
•: with regard to the transfer of common units, please read "Description of the Common Units—Transfer of Common Units."
•: with regard to allocations of taxable income and taxable loss, please read "Material Tax Consequences."

Organization and Duration

We were organized on January 25, 2002 and will have a perpetual existence.

Purpose

Our purpose under the partnership agreement is limited to serving as a member of the operating company and engaging in any business activities that may be engaged in by the operating company or that are approved by our general partner. All of our operations are conducted through our operating company, MarkWest Energy Operating Company, L.L.C., and its subsidiaries. We own 100% of the outstanding membership interest of the operating company. The limited liability company agreement of the operating company provides that the operating company may, directly or indirectly, engage in:

•: its operations as conducted immediately before our initial public offering;
•: any other activity approved by our general partner but only to the extent that our general partner reasonably determines that, as of the date of the acquisition or commencement of the activity, the activity generates "qualifying income" as this term is defined in Section 7704 of the Internal Revenue Code; or
•: any activity that enhances the operations of an activity that is described in either of the two preceding clauses or any other activity provided such activity does not affect our treatment as a partnership for Federal income tax purposes.

Our general partner is authorized in general to perform all acts deemed necessary to carry out our purposes and to conduct our business.

Power of Attorney

Each limited partner, and each person who acquires a unit from a unitholder and executes and delivers a transfer application, grants to our general partner and, if appointed, a liquidator, a power of attorney to, among other things, execute and file documents required for our qualification, continuance or dissolution. The power of attorney also grants our general partner the authority to amend, and to make consents and waivers under, the partnership agreement.

Capital Contributions

Unitholders are not obligated to make additional capital contributions, except as described below under "—Limited Liability."

Limited Liability

Assuming that a limited partner does not participate in the control of our business within the meaning of the Delaware Act and that he otherwise acts in conformity with the provisions of the partnership agreement, his liability under the Delaware Act will be limited, subject to possible exceptions, to the amount of capital he is obligated to contribute to us for his common units plus his share of any undistributed profits and assets. If it were determined, however, that the right, or exercise of the right, by the limited partners as a group:

•: to remove or replace our general partner;
•: to approve some amendments to the partnership agreement; or
•: to take other action under the partnership agreement;

constituted "participation in the control" of our business for the purposes of the Delaware Act, then the limited partners could be held personally liable for our obligations under the laws of Delaware, to the same extent as our general partner. This liability would extend to persons who transact business with us who reasonably believe that the limited partner is a general partner. Neither the partnership agreement nor the Delaware Act specifically provides for legal recourse against our general partner if a limited partner were to lose limited liability through any fault of our general partner. While this does not mean that a limited partner could not seek legal recourse, we know of no precedent for this type of a claim in Delaware case law.

Under the Delaware Act, a limited partnership may not make a distribution to a partner if, after the distribution, all liabilities of the limited partnership, other than liabilities to partners on account of their partnership interests and liabilities for which the recourse of creditors is limited to specific property of the Partnership, would exceed the fair value of the assets of the limited partnership. For the purpose of determining the fair value of the assets of a limited partnership, the Delaware Act provides that the fair value of property subject to liability for which recourse of creditors is limited shall be included in the assets of the limited partnership only to the extent that the fair value of that property exceeds the nonrecourse liability. The Delaware Act provides that a limited partner who receives a distribution and knew at the time of the distribution that the distribution was in violation of the Delaware Act shall be liable to the limited partnership for the amount of the distribution for three years. Under the Delaware Act, an assignee who becomes a substituted limited partner of a limited partnership is liable for the obligations of his assignor to make contributions to us, except the assignee is not obligated for liabilities unknown to him at the time he became a limited partner and that could not be ascertained from the partnership agreement.

Our subsidiaries conduct business in multiple states. Maintenance of our limited liability as a member of the operating company may require compliance with legal requirements in the jurisdictions in which the operating company conducts business, including qualifying our subsidiaries to do business there. Limitations on the liability of members for the obligations of a limited liability company have not been clearly established in many jurisdictions. If, by virtue of our membership interest in the operating company or otherwise, it were determined that we were conducting business in any state without compliance with the applicable limited partnership or limited liability company statute, or that the right or exercise of the right by the limited partners as a group to remove or replace our general partner, to approve some amendments to the partnership agreement, or to take other action under the partnership agreement constituted "participation in the control" of our business for purposes of the statutes of any relevant jurisdiction, then the limited partners could be held personally liable for our obligations under the law of that jurisdiction to the same extent as our general partner under the circumstances. We will operate in a manner that our general partner considers reasonable and necessary or appropriate to preserve the limited liability of the limited partners.

Voting Rights

The following matters require the unitholder vote specified below. Matters requiring the approval of a "unit majority" require:

•: during the subordination period, the approval of a majority of the common units, excluding those common units held by our general partner and its affiliates, and a majority of the subordinated units, voting as separate classes; and
•: after the subordination period, the approval of a majority of the common units.

Issuance of additional common units or units of equal rank with the common units during the subordination period		Unit majority, with certain exceptions described under "—Issuance of Additional Securities."
Issuance of units senior to the common units during the subordination period		Unit majority.
Issuance of units junior to the common units during the subordination period		No approval right.
Issuance of additional units after the subordination period		No approval rights.
Amendment of the partnership agreement		Certain amendments may be made by the general partner without the approval of the unitholders. Other amendments generally require the approval of a unit majority. See "—Amendment of the Partnership Agreement."
Merger of our partnership or the sale of all or substantially all of our assets		Unit majority. See "—Merger, Sale or Other Disposition of Assets."
Amendment of the limited liability company agreement and other action taken by us as sole member of the operating company		Unit majority if such amendment or other action would adversely affect our limited partners (or any particular class of limited partners) in any material respect. See "—Action Relating to the Operating Company."
Dissolution of our partnership		Unit majority. See "—Termination and Dissolution."
Reconstitution of our partnership upon dissolution		Unit majority. See "—Termination and Dissolution."

Withdrawal of the general partner		Under most circumstances, the approval of a majority of the common units, excluding common units held by the general partner and its affiliates, is required for the withdrawal of the general partner prior to June 30, 2012 in a manner which would cause a dissolution of our partnership. See "—Withdrawal or Removal of our General Partner."
Removal of the general partner		Not less than 66²/3% of the outstanding units, voting as a single class, including units held by our general partner and its affiliates. See "—Withdrawal or Removal of our General Partner."
Transfer of the general partner interest		Our general partner may transfer all, but not less than all, of its general partner interest in us without a vote of our unitholders to an affiliate or another person in connection with its merger or consolidation with or into, or sale of all or substantially all of its assets to such person. The approval of a majority of the common units, excluding common units held by the general partner and its affiliates, is required in other circumstances for a transfer of the general partner interest to a third party prior to June 30, 2012. See "—Transfer of General Partner Interests."
Transfer of incentive distribution rights		Except for transfers to an affiliate or another person as part of the general partner's merger or consolidation with or into, or sale of all or substantially all of its assets to or sale of all or substantially all its equity interests to such person, the approval of a majority of the common units, excluding common units held by our general partner and its affiliates, voting separately as a class, is required in most circumstances for a transfer of the incentive distribution rights to a third party prior to June 30, 2012. See "—Transfer of Incentive Distribution Rights."
Transfer of ownership interests in the general partner		No approval required at any time. See "—Transfer of Ownership Interests in the General Partner."

Issuance of Additional Securities

The partnership agreement authorizes us to issue an unlimited number of additional partnership securities and rights to buy partnership securities for the consideration and on the terms and conditions established by our general partner in its sole discretion without the approval of the unitholders. During the subordination period, however, except as we discuss in the following paragraph, we may not issue equity securities ranking senior to the common units or an aggregate of more than 1,207,500 additional common units or units on a parity with the common units, in each case, without the approval of the holders of a majority of the outstanding common units and subordinated units, voting as separate classes.

During or after the subordination period, we may issue an unlimited number of common units without the approval of the unitholders as follows:

•: upon exercise of an underwriters' over-allotment option;
•: upon conversion of the subordinated units;
•: under employee benefit plans;

•: upon conversion of the general partner interest and incentive distribution rights as a result of a withdrawal of our general partner;
•: in the event of a combination or subdivision of common units;
•: in connection with an acquisition or a capital improvement that increases cash flow from operations per unit on a pro forma basis; or
•: if the proceeds of the issuance are used exclusively to repay indebtedness the cost of which to service is greater than the distribution obligations associated with the units issued in connection with its retirement.

It is possible that we will fund acquisitions through the issuance of additional common units or other equity securities. Holders of any additional common units we issue will be entitled to share equally with the then-existing holders of common units in our distributions of available cash. In addition, the issuance of additional partnership interests may dilute the value of the interests of the then-existing holders of common units in our net assets.

In accordance with Delaware law and the provisions of our partnership agreement, we may also issue additional partnership securities interests that, in the sole discretion of our general partner, have special voting rights to which the common units are not entitled.

Upon issuance of additional partnership securities, our general partner will be required to make additional capital contributions to the extent necessary to maintain its 2% general partner interest in us. Moreover, our general partner will have the right, which it may from time to time assign in whole or in part to any of its affiliates, to purchase common units, subordinated units or other equity securities whenever, and on the same terms that, we issue those securities to persons other than our general partner and its affiliates, to the extent necessary to maintain its percentage interest, including its interest represented by common units and subordinated units, that existed immediately prior to each issuance. The holders of common units will not have preemptive rights to acquire additional common units or other partnership securities.

Amendment of the Partnership Agreement

General. Amendments to the partnership agreement may be proposed only by or with the consent of our general partner, which consent may be given or withheld in its sole discretion, except as discussed below. In order to adopt a proposed amendment, other than the amendments discussed below, our general partner must seek written approval of the holders of the number of units required to approve the amendment or call a meeting of the limited partners to consider and vote upon the proposed amendment. Except as we describe below, an amendment must be approved by a unit majority.

Prohibited Amendments. No amendment may be made that would:

•: enlarge the obligations of any limited partner without its consent, unless approved by at least a majority of the type or class of limited partner interests so affected;
•: enlarge the obligations of, restrict in any way any action by or rights of, or reduce in any way the amounts distributable, reimbursable or otherwise payable by us to our general partner or any of its affiliates without the consent of our general partner, which may be given or withheld in its sole discretion;
•: change the term of our partnership;
•: provide that our partnership is not dissolved upon an election to dissolve our partnership by our general partner that is approved by a unit majority; or

•: give any person the right to dissolve our partnership other than our general partner's right to dissolve our partnership with the approval of a unit majority.

The provision of the partnership agreement preventing the amendments having the effects described in any of the clauses above can be amended upon the approval of the holders of at least 90% of the outstanding units voting together as a single class.

No Unitholder Approval. Our general partner may generally make amendments to the partnership agreement without the approval of any limited partner or assignee to reflect:

•: a change in our name, the location of our principal place of business, our registered agent or our registered office;
•: the admission, substitution, withdrawal, or removal of partners in accordance with the partnership agreement;
•: a change that, in the sole discretion of our general partner, is necessary or advisable for us to qualify or to continue our qualification as a limited partnership or a partnership in which the limited partners have limited liability under the laws of any state or to ensure that neither we, the operating company nor its subsidiaries will be treated as an association taxable as a corporation or otherwise taxed as an entity for federal income tax purposes;
•: an amendment that is necessary, in the opinion of our counsel, to prevent us or our general partner or its directors, officers, agents, or trustees from in any manner being subjected to the provisions of the Investment Company Act of 1940, the Investment Advisors Act of 1940, or plan asset regulations adopted under the Employee Retirement Income Security Act of 1974, whether or not substantially similar to plan asset regulations currently applied or proposed;
•: subject to the limitations on the issuance of additional partnership securities described above, an amendment that in the discretion of our general partner is necessary or advisable for the authorization of additional partnership securities or rights to acquire partnership securities;
•: any amendment expressly permitted in the partnership agreement to be made by our general partner acting alone;
•: an amendment effected, necessitated or contemplated by a merger agreement that has been approved under the terms of the partnership agreement;
•: any amendment that, in the discretion of our general partner, is necessary or advisable for the formation by us of, or our investment in, any corporation, partnership or other entity, as otherwise permitted by the partnership agreement;
•: a change in our fiscal year or taxable year and related changes; or
•: any other amendments substantially similar to any of the matters described in the clauses above.

In addition, our general partner may make amendments to the partnership agreement without the approval of any limited partner or assignee if those amendments, in the discretion of our general partner:

•: do not adversely affect the limited partners (or any particular class of limited partners) in any material respect;
•: are necessary or advisable to satisfy any requirements, conditions or guidelines contained in any opinion, directive, order, ruling or regulation of any federal or state agency or judicial authority or contained in any federal or state statute;
•: are necessary or advisable to facilitate the trading of limited partner interests or to comply with any rule, regulation, guideline or requirement of any securities exchange on which the limited

partner interests are or will be listed for trading, compliance with any of which our general partner deems to be in our best interest and the best interest of the limited partners;

•: are necessary or advisable for any action taken by our general partner relating to splits or combinations of units under the provisions of the partnership agreement; or
•: are required to effect the intent expressed in this prospectus or the intent of the provisions of the partnership agreement or are otherwise contemplated by the partnership agreement.

Opinion of Counsel and Unitholder Approval. Our general partner will not be required to obtain an opinion of counsel that an amendment will not result in a loss of limited liability to the limited partners or result in our being treated as an entity for federal income tax purposes if one of the amendments described above under "—No Unitholder Approval" should occur. No other amendments to the partnership agreement will become effective without the approval of holders of at least 90% of the units unless we obtain an opinion of counsel to the effect that the amendment will not affect the limited liability under applicable law of any of our limited partners or cause us, the operating company or its subsidiaries to be taxable as a corporation or otherwise to be taxed as an entity for federal income tax purposes (to the extent not previously taxed as such).

Any amendment that would have a material adverse effect on the rights or preferences of any type or class of outstanding units in relation to other classes of units will require the approval of at least a majority of the type or class of units so affected. Any amendment that reduces the voting percentage required to take any action must be approved by the affirmative vote of limited partners constituting not less than the voting requirement sought to be reduced.

Action Relating to the Operating Company

Without the approval of the holders of units representing a unit majority, our general partner is prohibited from consenting on our behalf, as the sole member of the operating company, to any amendment to the limited liability company agreement of the operating company or taking any action on our behalf permitted to be taken by a member of the operating company in each case that would adversely affect our limited partners (or any particular class of limited partners) in any material respect.

Merger, Sale or Other Disposition of Assets

The partnership agreement generally prohibits our general partner, without the prior approval of the holders of units representing a unit majority, from causing us to, among other things, sell, exchange or otherwise dispose of all or substantially all of our assets in a single transaction or a series of related transactions, including by way of merger, consolidation or other combination, or approving on our behalf the sale, exchange or other disposition of all or substantially all of the assets of our subsidiaries as a whole. Our general partner may, however, mortgage, pledge, hypothecate or grant a security interest in all or substantially all of our assets without that approval. Our general partner may also sell all or substantially all of our assets under a foreclosure or other realization upon those encumbrances without that approval.

If conditions specified in the partnership agreement are satisfied, our general partner may merge us or any of our subsidiaries into, or convey some or all of our assets to, a newly formed entity if the sole purpose of that merger or conveyance is to change our legal form into another limited liability entity. The unitholders are not entitled to dissenters' rights of appraisal under the partnership agreement or applicable Delaware law in the event of a merger or consolidation, a sale of substantially all of our assets or any other transaction or event for such purpose.

Termination and Dissolution

We will continue as a limited partnership until terminated under the partnership agreement. We will dissolve upon:

•: the election of our general partner to dissolve us, if approved by the holders of units representing a unit majority;
•: the sale, exchange or other disposition of all or substantially all of our assets and properties and our subsidiaries;
•: the entry of a decree of judicial dissolution of the Partnership; or
•: the withdrawal or removal of our general partner or any other event that results in its ceasing to be our general partner other than by reason of a transfer of its general partner interest in accordance with the partnership agreement or withdrawal or removal of the general partner following approval and admission of a successor.

Upon a dissolution under the last clause, the holders of a majority of the outstanding common units and subordinated units, voting as separate classes, may also elect, within specific time limitations, to reconstitute us and continue our business on the same terms and conditions described in the partnership agreement by forming a new limited partnership on terms identical to those in the partnership agreement and having as general partner an entity approved by the holders of a majority of the outstanding common units and subordinated units, voting as separate classes, subject to our receipt of an opinion of counsel to the effect that:

•: the action would not result in the loss of limited liability of any limited partner; and
•: neither we, the reconstituted limited partnership nor the operating company would be treated as an association taxable as a corporation or otherwise be taxable as an entity for federal income tax purposes upon the exercise of that right to continue.

Liquidation and Distribution of Proceeds

Upon our dissolution, unless we are reconstituted and continued as a new limited partnership, the liquidator authorized to wind up our affairs will, acting with all of the powers of our general partner that the liquidator deems necessary or desirable in its judgment, liquidate our assets and apply the proceeds of the liquidation as provided in "Cash Distribution Policy—Distributions of Cash upon Liquidation." The liquidator may defer liquidation of our assets for a reasonable period or distribute assets to partners in kind if it determines that a sale would be impractical or would cause undue loss to the partners.

Withdrawal or Removal of our General Partner

Except as described below, our general partner has agreed not to withdraw voluntarily as our general partner prior to June 30, 2012 without obtaining the approval of the holders of at least a majority of the outstanding common units, excluding common units held by our general partner and its affiliates, and furnishing an opinion of counsel regarding limited liability and tax matters. On or after June 30, 2012, our general partner may withdraw as general partner without first obtaining approval of any unitholder by giving 90 days' written notice, and that withdrawal will not constitute a violation of the partnership agreement. Notwithstanding the foregoing, our general partner may withdraw without unitholder approval upon 90 days' notice to the limited partners if at least 50% of the outstanding common units are held or controlled by one person and its affiliates other than our general partner and its affiliates. In addition, the partnership agreement permits our general partner in some instances to sell or otherwise transfer all of its general partner interest in us without the approval of the unitholders. Please read "—Transfer of General Partner Interests."

Upon the withdrawal of our general partner under any circumstances, other than as a result of a transfer by our general partner of all or a part of its general partner interest in us, the holders of a majority of the outstanding common units and subordinated units, voting as separate classes, may select a successor to that withdrawing general partner. If a successor is not elected, or is elected but an opinion of counsel regarding limited liability and tax matters cannot be obtained, we will be dissolved, wound up and liquidated, unless within 180 days after that withdrawal, the holders of a majority of the outstanding common units and subordinated units, voting as separate classes, agree in writing to continue our business and to appoint a successor general partner. Please read "—Termination and Dissolution."

Our general partner may not be removed unless that removal is approved by the vote of the holders of not less than 66²/3% of the outstanding units, including units held by our general partner and its affiliates, and we receive an opinion of counsel regarding limited liability and tax matters. Any removal of our general partner is also subject to the approval of a successor general partner by the vote of the holders of a majority of the outstanding common units and subordinated units, voting as separate classes. The ownership of more than 33¹/3% of the outstanding units by our general partner and its affiliates would give it the practical ability to prevent its removal. As of September 30, 2006, affiliates of our general partner owned approximately 17% of the outstanding units.

The partnership agreement also provides that if our general partner is removed under circumstances where cause does not exist and units held by our general partner and its affiliates are not voted in favor of that removal:

•: the subordination period will end and all outstanding subordinated units will immediately convert into common units on a one-for-one basis;
•: any existing arrearages in payment of the minimum quarterly distribution on the common units will be extinguished; and
•: our general partner will have the right to convert its general partner interest and its incentive distribution rights into common units or to receive cash in exchange for those interests based on the fair market value of those interests at the time.

In the event of removal of a general partner under circumstances where cause exists or withdrawal of a general partner where that withdrawal violates the partnership agreement, a successor general partner will have the option to purchase the general partner interest and incentive distribution rights of the departing general partner for a cash payment equal to the fair market value of those interests. Under all other circumstances where our general partner withdraws or is removed by the limited partners, the departing general partner will have the option to require the successor general partner to purchase the general partner interest of the departing general partner and its incentive distribution rights for the fair market value. In each case, this fair market value will be determined by agreement between the departing general partner and the successor general partner. If no agreement is reached, an independent investment banking firm or other independent expert selected by the departing general partner and the successor general partner will determine the fair market value. Or, if the departing general partner and the successor general partner cannot agree upon an expert, then an expert chosen by agreement of the experts selected by each of them will determine the fair market value.

If the option described above is not exercised by either the departing general partner or the successor general partner, the departing general partner's general partner interest and its incentive distribution rights will automatically convert into common units equal to the fair market value of those interests as determined by an investment banking firm or other independent expert selected in the manner described in the preceding paragraph.

In addition, we will be required to reimburse the departing general partner for all amounts due the departing general partner, including, without limitation, all employee-related liabilities, including

severance liabilities, incurred for the termination of any employees employed by the departing general partner or its affiliates for our benefit.

Transfer of General Partner Interests

Except for transfer by our general partner of all, but not less than all, of its general partner interest in us to:

•: an affiliate of our general partner (other than an individual); or
•: another entity as part of the merger or consolidation of our general partner with or into another entity or the transfer by our general partner of all or substantially all of its assets to another entity,

our general partner may not transfer all or any part of its general partner interest in us to another person prior to June 30, 2012 without the approval of the holders of at least a majority of the outstanding common units, excluding common units held by our general partner and its affiliates. As a condition of this transfer, the transferee must, among other things, assume the rights and duties of our general partner, agree to be bound by the provisions of the partnership agreement and furnish an opinion of counsel regarding limited liability and tax matters.

Our general partner and its affiliates may at any time transfer units to one or more persons, without unitholder approval, except that they may not transfer subordinated units to us.

Transfer of Ownership Interests in General Partner

At any time, the members of our general partner may sell or transfer all or part of their membership interests in our general partner without the approval of the unitholders.

Transfer of Incentive Distribution Rights

Our general partner or its affiliates or a subsequent holder of the incentive distribution rights may transfer its incentive distribution rights to an affiliate or another person as part of its merger or consolidation with or into, or sale of all or substantially all of its assets to, or a sale of all or substantially all of its equity interests to, that person without the prior approval of the unitholders; but, in each case, the transferee must agree to be bound by the provisions of the partnership agreement. Prior to June 30, 2012, other transfers of the incentive distribution rights will require the affirmative vote of holders of a majority of the outstanding common units, excluding common units held by the general partner and its affiliates. On or after June 30, 2012, the incentive distribution rights will be freely transferable.

Change of Management Provisions

The partnership agreement contains specific provisions that are intended to discourage a person or group from attempting to remove MarkWest Energy GP, L.L.C. as our general partner or otherwise change management. If any person or group other than our general partner and its affiliates acquires beneficial ownership of 20% or more of any class of units, that person or group loses voting rights on all of its units. This loss of voting rights does not apply to any person or group that acquires the units from our general partner or its affiliates and any transferees of that person or group approved by our general partner or to any person or group who acquires the units with the prior approval of the board of directors.

•: the subordination period will end and all outstanding subordinated units will immediately convert into common units on a one-for-one basis;
•: any existing arrearages in payment of the minimum quarterly distribution on the common units will be extinguished; and
•: our general partner will have the right to convert its general partner interest and its incentive distribution rights into common units or to receive cash in exchange for those interests.

Limited Call Right

If at any time our general partner and its affiliates hold more than 80% of the then-issued and outstanding partnership securities of any class, our general partner will have the right, which it may assign in whole or in part to any of its affiliates or to us, to acquire all, but not less than all, of the remaining partnership securities of the class held by unaffiliated persons as of a record date to be selected by our general partner, on at least ten but not more than 60 days notice. The purchase price in the event of this purchase is the greater of:

•: the highest cash price paid by either of our general partner or any of its affiliates for any partnership securities of the class purchased within the 90 days preceding the date on which our general partner first mails notice of its election to purchase those partnership securities; and
•: the current market price as of the date three days before the date the notice is mailed.

As a result of our general partner's right to purchase outstanding partnership securities, a holder of partnership securities may have his partnership securities purchased at an undesirable time or price. The tax consequences to a unitholder of the exercise of this call right are the same as a sale by that unitholder of his common units in the market. Please read "Material Tax Consequences—Disposition of Common Units."

Meetings; Voting

Except as described below regarding a person or group owning 20% or more of any class of units then outstanding, unitholders or assignees who are record holders of units on the record date will be entitled to notice of, and to vote at, meetings of our limited partners and to act upon matters for which approvals may be solicited. Common units that are owned by an assignee who is a record holder, but who has not yet been admitted as a limited partner, will be voted by our general partner at the written direction of the record holder. Absent direction of this kind, the common units will not be voted, except that, in the case of common units held by our general partner on behalf of non-citizen assignees, our general partner will distribute the votes on those common units in the same ratios as the votes of limited partners on other units are cast.

Our general partner does not anticipate that any meeting of unitholders will be called in the foreseeable future. Any action that is required or permitted to be taken by the unitholders may be taken either at a meeting of the unitholders or without a meeting if consents in writing describing the action so taken are signed by holders of the number of units necessary to authorize or take that action at a meeting. Meetings of the unitholders may be called by our general partner or by unitholders owning at least 20% of the outstanding units of the class for which a meeting is proposed. Unitholders may vote either in person or by proxy at meetings. The holders of a majority of the outstanding units of the class or classes for which a meeting has been called, represented in person or by proxy, will

constitute a quorum unless any action by the unitholders requires approval by holders of a greater percentage of the units, in which case the quorum will be the greater percentage.

Each record holder of a unit has a vote according to his percentage interest in us, although additional limited partner interests having special voting rights could be issued. Please read "—Issuance of Additional Securities." However, if at any time any person or group, other than our general partner and its affiliates, or a direct or subsequently approved transferee of our general partner or its affiliates, acquires, in the aggregate, beneficial ownership of 20% or more of any class of units then outstanding, that person or group will lose voting rights on all of its units and the units may not be voted on any matter and will not be considered to be outstanding when sending notices of a meeting of unitholders, calculating required votes, determining the presence of a quorum or for other similar purposes. Common units held in nominee or street name account will be voted by the broker or other nominee in accordance with the instruction of the beneficial owner unless the arrangement between the beneficial owner and his nominee provides otherwise. Except as the partnership agreement otherwise provides, subordinated units will vote together with common units as a single class.

Any notice, demand, request, report or proxy material required or permitted to be given or made to record holders of common units under the partnership agreement will be delivered to the record holder by us or by the transfer agent.

Status as Limited Partner or Assignee

Except as described under "—Limited Liability," the common units will be fully paid, and unitholders will not be required to make additional contributions.

An assignee of a common unit, after executing and delivering a transfer application, but pending its admission as a substituted limited partner, is entitled to an interest equivalent to that of a limited partner for the right to share in allocations and distributions from us, including liquidating distributions. Our general partner will vote and exercise other powers attributable to common units owned by an assignee that has not become a substitute limited partner at the written direction of the assignee. See "—Meetings; Voting." Transferees that do not execute and deliver a transfer application will not be treated as assignees nor as record holders of common units, and will not receive cash distributions, federal income tax allocations or reports furnished to holders of common units. Please read "Description of the Common Units—Transfer of Common Units."

Non-citizen Assignees; Redemption

If we are or become subject to federal, state or local laws or regulations that, in the reasonable determination of our general partner, create a substantial risk of cancellation or forfeiture of any property that we have an interest in because of the nationality, citizenship or other related status of any limited partner or assignee, we may redeem the units held by the limited partner or assignee at their current market price. In order to avoid any cancellation or forfeiture, our general partner may require each limited partner or assignee to furnish information about his nationality, citizenship or related status. If a limited partner or assignee fails to furnish information about his nationality, citizenship or other related status within 30 days after a request for the information or our general partner determines after receipt of the information that the limited partner or assignee is not an eligible citizen, the limited partner or assignee may be treated as a non-citizen assignee. In addition to other limitations on the rights of an assignee that is not a substituted limited partner, a non-citizen assignee does not have the right to direct the voting of his units and may not receive distributions in kind upon our liquidation.

Indemnification

Under the partnership agreement, in most circumstances, we will indemnify the following persons, to the fullest extent permitted by law, from and against all losses, claims, damages or similar events:

•: our general partner;
•: any departing general partner;
•: any person who is or was an affiliate of a general partner or any departing general partner;
•: any person who is or was a member, partner, officer, director, employee, agent or trustee of our general partner or any departing general partner or any affiliate of a general partner or any departing general partner; or
•: any person who is or was serving at the request of a general partner or any departing general partner or any affiliate of a general partner or any departing general partner as an officer, director, employee, member, partner, agent or trustee of another person.

Any indemnification under these provisions will only be out of our assets. Our general partner and its affiliates will not be personally liable for, or have any obligation to contribute or loan funds or assets to us to enable us to effectuate, indemnification. We may purchase insurance against liabilities asserted against and expenses incurred by persons for our activities, regardless of whether we would have the power to indemnify the person against liabilities under the partnership agreement.

Books and Reports

Our general partner is required to keep appropriate books of our business at our principal offices. The books will be maintained for both tax and financial reporting purposes on an accrual basis. For tax and fiscal reporting purposes, our fiscal year is the calendar year.

We will furnish or make available to record holders of common units, within 120 days after the close of each fiscal year, an annual report containing audited financial statements and a report on those financial statements by our independent public accountants. Except for our fourth quarter, we will also furnish or make available summary financial information within 90 days after the close of each quarter.

We will furnish each record holder of a unit with information reasonably required for tax reporting purposes within 90 days after the close of each calendar year. This information is expected to be furnished in summary form so that some complex calculations normally required of partners can be avoided. Our ability to furnish this summary information to unitholders will depend on the cooperation of unitholders in supplying us with specific information. Every unitholder will receive information to assist him in determining his federal and state tax liability and filing his federal and state income tax returns, regardless of whether he supplies us with information.

Right to Inspect our Books and Records

The partnership agreement provides that a limited partner can, for a purpose reasonably related to his interest as a limited partner, upon reasonable demand and at his own expense, have furnished to him:

•: a current list of the name and last known address of each partner;
•: a copy of our tax returns;
•: information as to the amount of cash, and a description and statement of the agreed value of any other property or services, contributed or to be contributed by each partner and the date on which each became a partner;

•: copies of the partnership agreement, the certificate of limited partnership of the Partnership, related amendments and powers of attorney under which they have been executed;
•: information regarding the status of our business and financial condition; and
•: any other information regarding our affairs as is just and reasonable.

Our general partner may, and intends to, keep confidential from the limited partners trade secrets or other information the disclosure of which our general partner believes in good faith is not in our best interests or which we are required by law or by agreements with third parties to keep confidential.

Registration Rights

Under the partnership agreement, we have agreed to register for resale under the Securities Act of 1933 and applicable state securities laws any common units, subordinated units or other partnership securities proposed to be sold by our general partner or any of its affiliates (including MarkWest Hydrocarbon, its officers and the officers, directors and owners of our general partner) or their assignees if an exemption from the registration requirements is not otherwise available. These registration rights continue for two years following any withdrawal or removal of MarkWest Energy GP, L.L.C. as our general partner. We are obligated to pay all expenses incidental to the registration, excluding underwriting discounts and commissions. Please read "Units Eligible for Future Sale."

MATERIAL TAX CONSEQUENCES

This section is a summary of the material tax consequences that may be relevant to prospective unitholders who are individual citizens or residents of the United States and, unless otherwise noted in the following discussion, is the opinion of Vinson & Elkins L.L.P., counsel to our general partner and us, insofar as it relates to matters of United States federal income tax law and legal conclusions with respect to those matters. This section is based on current provisions of the Internal Revenue Code, existing and proposed regulations to the extent noted and current administrative rulings and court decisions, all of which are subject to change. Later changes in these authorities may cause the tax consequences to vary substantially from the consequences described below. Unless the context otherwise requires, references in this section to "us" or "we" are to MarkWest Energy Partners and the operating partnership.

This section does not address all federal income tax matters that affect us or the unitholders. Furthermore, this section focuses on unitholders who are individual citizens or residents of the United States and has only limited application to corporations, estates, trusts, non-resident aliens or other unitholders subject to specialized tax treatment, such as tax-exempt institutions, foreign persons, individual retirement accounts (IRAs), real estate investment trusts (REITs) or mutual funds. Accordingly, each prospective unitholder is urged to consult, and depend on, his own tax advisor in analyzing the federal, state, local and foreign tax consequences particular to him of the ownership or disposition of common units.

All statements as to matters of law and legal conclusions, but not as to factual matters, contained in this section, unless otherwise noted, are the opinion of Vinson & Elkins L.L.P. and are based on the accuracy of the representations made by us.

No ruling has been or will be requested from the IRS regarding any matter that affects us or prospective unitholders. Instead, we will rely on opinions and advice of Vinson & Elkins L.L.P. Unlike a ruling, an opinion of counsel represents only that counsel's best legal judgment and does not bind the IRS or the courts. Accordingly, the opinions and statements made here may not be sustained by a court if contested by the IRS. Any contest of this sort with the IRS may materially and adversely impact the market for the common units and the prices at which common units trade. In addition, the costs of any contest with the IRS, principally legal, accounting and related fees, will result in a reduction in cash available for distribution to our unitholders and our general partner and thus will be borne directly or indirectly by the unitholders and the general partner. Furthermore, the tax treatment of us, or of an investment in us, may be significantly modified by future legislative or administrative changes or court decisions. Any modifications may or may not be retroactively applied.

For the reasons described below, Vinson & Elkins L.L.P. has not rendered an opinion with respect to the following specific federal income tax issues:

(1) the treatment of a unitholder whose common units are loaned to a short seller to cover a short sale of common units (please read "—Tax Consequences of Unit Ownership—Treatment of Short Sales");

(2) whether our monthly convention for allocating taxable income and losses is permitted by existing Treasury Regulations (please read "—Disposition of Common Units—Allocations Between Transferors and Transferees"); and

(3) whether our method for depreciating Section 743 adjustments is sustainable in certain cases (please read "—Tax Consequences of Unit Ownership—Section 754 Election").

Partnership Status

A partnership is not a taxable entity and incurs no federal income tax liability. Instead, each partner of a partnership is required to take into account his share of items of income, gain, loss and deduction of the partnership in computing his federal income tax liability, even if no cash distributions are made to him by the partnership. Distributions by a partnership to a partner are generally not taxable to the partner unless the amount of cash distributed to him is in excess of his adjusted basis in his partnership interest.

Section 7704 of the Internal Revenue Code provides that publicly-traded partnerships will, as a general rule, be taxed as corporations. However, an exception, referred to as the "Qualifying Income Exception," exists with respect to publicly-traded partnerships of which 90% or more of the gross income for every taxable year consists of "qualifying income." Qualifying income includes income and gains derived from the transportation, storage and processing of crude oil, natural gas and products thereof and fertilizer. Other types of qualifying income include interest (other than from a financial business), dividends, gains from the sale of real property and gains from the sale or other disposition of capital assets held for the production of income that otherwise constitutes qualifying income. We estimate that less than 4% of our current gross income is not qualifying income; however, this estimate could change from time to time. Based upon and subject to this estimate, the factual representations made by us and the general partner and a review of the applicable legal authorities, Vinson & Elkins L.L.P. is of the opinion that at least 90% of our current gross income constitutes qualifying income.

No ruling has been or will be sought from the IRS and the IRS has made no determination as to our status or the status of the operating partnership for federal income tax purposes or whether our operations generate "qualifying income" under Section 7704 of the Internal Revenue Code. Instead, we will rely on the opinion of Vinson & Elkins L.L.P. that, based on the Internal Revenue Code, its regulations, published revenue rulings and court decisions and the representations set forth below, MarkWest Energy Partners will be classified as partnerships for federal income tax purposes.

In rendering its opinion, Vinson & Elkins L.L.P. has relied on factual representations made by us and the general partner. The representations made by us and our general partner upon which counsel has relied are:

(a) Neither we nor the operating partnership has elected or will elect to be treated as a corporation; and

(b) For each taxable year, more than 90% of our gross income has been and will be income that Vinson & Elkins L.L.P. has opined or will opine is "qualifying income" within the meaning of Section 7704(d) of the Internal Revenue Code.

If we fail to meet the Qualifying Income Exception, other than a failure that is determined by the IRS to be inadvertent and that is cured within a reasonable time after discovery, in which case the IRS may also require us to make adjustments with respect to our unitholders or pay other amounts, we will be treated as if we had transferred all of our assets, subject to liabilities, to a newly formed corporation, on the first day of the year in which we fail to meet the Qualifying Income Exception, in return for stock in that corporation, and then distributed that stock to the unitholders in liquidation of their interests in us. This deemed contribution and liquidation should be tax-free to unitholders and us so long as we, at that time, do not have liabilities in excess of the tax basis of our assets. Thereafter, we would be treated as a corporation for federal income tax purposes.

If we were taxable as a corporation in any taxable year, either as a result of a failure to meet the Qualifying Income Exception or otherwise, our items of income, gain, loss and deduction would be reflected only on our tax return rather than being passed through to the unitholders, and our net income would be taxed to us at corporate rates. In addition, any distribution made to a unitholder would be treated as either taxable dividend income, to the extent of our current or accumulated

earnings and profits, or, in the absence of earnings and profits, a nontaxable return of capital, to the extent of the unitholder's tax basis in his common units, or taxable capital gain, after the unitholder's tax basis in his common units is reduced to zero. Accordingly, taxation as a corporation would result in a material reduction in a unitholder's cash flow and after-tax return and thus would likely result in a substantial reduction of the value of the units.

The remainder of this section is based on Vinson & Elkins L.L.P.'s opinion that MarkWest Energy Partners will be classified as a partnership for federal income tax purposes.

Limited Partner Status

Unitholders who have become limited partners of MarkWest Energy Partners will be treated as partners of MarkWest Energy Partners for federal income tax purposes. Also:

(a) assignees who have executed and delivered transfer applications, and are awaiting admission as limited partners, and

(b) unitholders whose common units are held in street name or by a nominee and who have the right to direct the nominee in the exercise of all substantive rights attendant to the ownership of their common units,

will be treated as partners of MarkWest Energy Partners for federal income tax purposes.

As there is no direct authority addressing the federal tax treatment of assignees of common units who are entitled to execute and deliver transfer applications and thereby become entitled to direct the exercise of attendant rights, but who fail to execute and deliver transfer applications, the opinion of Vinson & Elkins L.L.P. does not extend to these persons. Furthermore, a purchaser or other transferee of common units who does not execute and deliver a transfer application may not receive some federal income tax information or reports furnished to record holders of common units unless the common units are held in a nominee or street name account and the nominee or broker has executed and delivered a transfer application for those common units.

A beneficial owner of common units whose units have been transferred to a short seller to complete a short sale would appear to lose his status as a partner with respect to those units for federal income tax purposes. Please read "—Tax Consequences of Unit Ownership—Treatment of Short Sales."

Income, gain, deductions or losses would not appear to be reportable by a unitholder who is not a partner for federal income tax purposes, and any cash distributions received by a unitholder who is not a partner for federal income tax purposes would therefore appear to be fully taxable as ordinary income. These holders are urged to consult their own tax advisors with respect to their status as partners in MarkWest Energy Partners for federal income tax purposes.

Tax Consequences of Unit Ownership

Flow-Through of Taxable Income. We will not pay any federal income tax. Instead, each unitholder will be required to report on his income tax return his share of our income, gains, losses and deductions without regard to whether corresponding cash distributions are received by him. Consequently, we may allocate income to a unitholder even if he has not received a cash distribution. Each unitholder will be required to include in income his allocable share of our income, gains, losses and deductions for our taxable year ending with or within his taxable year. Our taxable year ends on December 31.

Treatment of Distributions. Cash distributions made by us to a unitholder generally will not be taxable to him for federal income tax purposes to the extent of his tax basis in his common units immediately before the distribution. Cash distributions made by us to a unitholder in an amount in

excess of his tax basis in his common units generally will be considered to be gain from the sale or exchange of the common units, taxable in accordance with the rules described under "—Disposition of Common Units" below. To the extent that cash distributions made by us cause a unitholder's "at risk" amount to be less than zero at the end of any taxable year, he must recapture any losses deducted in previous years. Please read "—Limitations on Deductibility of Losses."

Any reduction in a unitholder's share of our liabilities for which no partner, including the general partner, bears the economic risk of loss, known as "nonrecourse liabilities," will be treated as a distribution of cash to that unitholder. A decrease in a unitholder's percentage interest in us because of our issuance of additional common units will decrease his share of our nonrecourse liabilities, and thus will result in a corresponding deemed distribution of cash, which may constitute a non-pro rata distribution. A non-pro rata distribution of money or property may result in ordinary income to a unitholder, regardless of his tax basis in his common units, if the distribution reduces the unitholder's share of our "unrealized receivables," including depreciation recapture, and/or substantially appreciated "inventory items," both as defined in Section 751 of the Internal Revenue Code, and collectively, "Section 751 Assets." To that extent, he will be treated as having received his proportionate share of the Section 751 Assets and having exchanged those assets with us in return for the non-pro rata portion of the actual distribution made to him. This latter deemed exchange will generally result in the unitholder's realization of ordinary income. That income will equal the excess of (1) the non-pro rata portion of that distribution over (2) the unitholder's tax basis for the share of Section 751 Assets deemed relinquished in the exchange.

Basis of Common Units. A unitholder's initial tax basis for his common units will be the amount he paid for the common units plus his share of our nonrecourse liabilities. That basis will be increased by his share of our income and by any increases in his share of our nonrecourse liabilities. That basis will be decreased, but not below zero, by distributions to him from us, by his share of our losses, by any decreases in his share of our nonrecourse liabilities and by his share of our expenditures that are not deductible in computing taxable income and are not required to be capitalized. A unitholder will have no share of our debt that is recourse to the general partner, but will have a share, generally based on his share of profits, of our nonrecourse liabilities. Please read "—Disposition of Common Units—Recognition of Gain or Loss."

Limitations on Deductibility of Losses. The deduction by a unitholder of his share of our losses will be limited to the tax basis in his units and, in the case of an individual unitholder or a corporate unitholder, if more than 50% of the value of its stock is owned directly or indirectly by or for five or fewer individuals or certain tax-exempt organizations, to the amount for which the unitholder is considered to be "at risk" with respect to our activities, if that is less than his tax basis. A unitholder must recapture losses deducted in previous years to the extent that distributions cause his at-risk amount to be less than zero at the end of any taxable year. Losses disallowed to a unitholder or recaptured as a result of these limitations will carry forward and will be allowable as a deduction in a later year to the extent that his tax basis or at-risk amount, whichever is the limiting factor, is subsequently increased. Upon the taxable disposition of a unit, any gain recognized by a unitholder can be offset by losses that were previously suspended by the at risk limitation but may not be offset by losses suspended by the basis limitation. Any excess loss above that gain previously suspended by the at risk or basis limitations is no longer utilizable.

In general, a unitholder will be at risk to the extent of his tax basis in his units, excluding any portion of that basis attributable to his share of our nonrecourse liabilities, reduced by any amount of money he borrows to acquire or hold his units, if the lender of those borrowed funds owns an interest in us, is related to the unitholder or can look only to the units for repayment, or any portion of that basis representing amounts otherwise protected against loss because of a guarantee, stop loss agreement or other similar arrangement. A unitholder's at-risk amount will increase or decrease as the

tax basis of the unitholder's units increases or decreases, other than tax basis increases or decreases attributable to increases or decreases in his share of our nonrecourse liabilities.

The passive loss limitations generally provide that individuals, estates, trusts and some closely-held corporations and personal service corporations are permitted to deduct losses from passive activities, which are generally defined as corporate or partnership activities in which the taxpayer does not materially participate, only to the extent of the taxpayer's income from those passive activities. The passive loss limitations are applied separately with respect to each publicly-traded partnership. Consequently, any passive losses we generate will only be available to offset our passive income generated in the future and will not be available to offset income from other passive activities or investments, including our investments or a unitholder's investments in other publicly-traded partnerships, or salary or active business income. Similarly, a unitholder's share of our net income may not be offset by any other current or carryover losses from other passive activities, including those attributable to other publicly traded partnerships. Passive losses that are not deductible because they exceed a unitholder's share of income we generate may be deducted in full when he disposes of his entire investment in us in a fully taxable transaction with an unrelated party. The passive activity loss rules are applied after other applicable limitations on deductions, including the at-risk rules and the basis limitation.

Limitations on Interest Deductions. The deductibility of a non-corporate taxpayer's "investment interest expense" is generally limited to the amount of that taxpayer's "net investment income." Investment interest expense includes:

•: interest on indebtedness properly allocable to property held for investment;
•: our interest expense attributable to portfolio income; and
•: the portion of interest expense incurred to purchase or carry an interest in a passive activity to the extent attributable to portfolio income.

The computation of a unitholder's investment interest expense will take into account interest on any margin account borrowing or other loan incurred to purchase or carry a unit. Net investment income includes gross income from property held for investment and amounts treated as portfolio income under the passive loss rules, less deductible expenses, other than interest, directly connected with the production of investment income, but generally does not include gains attributable to the disposition of property held for investment. The IRS has indicated that net passive income earned by a publicly-traded partnership will be treated as investment income to its unitholders. In addition, the unitholder's share of our portfolio income will be treated as investment income.

Entity-Level Collections. If we are required or elect under applicable law to pay any federal, state or local income tax on behalf of any unitholder or the general partner or any former unitholder, we are authorized to pay those taxes from our funds. That payment, if made, will be treated as a distribution of cash to the unitholder on whose behalf the payment was made. If the payment is made on behalf of a unitholder whose identity cannot be determined, we are authorized to treat the payment as a distribution to all current unitholders. We are authorized to amend the partnership agreement in the manner necessary to maintain uniformity of intrinsic tax characteristics of units and to adjust later distributions, so that after giving effect to these distributions, the priority and characterization of distributions otherwise applicable under the partnership agreement is maintained as nearly as is practicable. Payments by us as described above could give rise to an overpayment of tax on behalf of a unitholder in which event the unitholder would be required to file a claim in order to obtain a credit or refund.

Allocation of Income, Gain, Loss and Deduction. In general, if we have a net profit, our items of income, gain, loss and deduction will be allocated among the general partner and the unitholders in

accordance with their percentage interests in us. At any time that distributions are made to the common units in excess of distributions to the subordinated units, or that incentive distributions are made to the general partner, gross income will be allocated to the recipients to the extent of those distributions. If we have a net loss for the entire year, that amount of loss will be allocated first to the general partner and the unitholders in accordance with their percentage interests in us to the extent of their positive capital accounts and then to our general partner.

Specified items of our income, gain, loss and deduction will be allocated to account for the difference between the tax basis and fair market value of our assets at the time of an offering, referred to in this discussion as "Contributed Property." The effect of these allocations to a unitholder who purchases common units in an offering will be essentially the same as if the tax basis of our assets were equal to their fair market value at the time of the offering. In addition, items of recapture income will be allocated to the extent possible to the unitholder who was allocated the deduction giving rise to the treatment of that gain as recapture income in order to minimize the recognition of ordinary income by other unitholders. Finally, although we do not expect that our operations will result in the creation of negative capital accounts, if negative capital accounts nevertheless result, items of our income and gain will be allocated in an amount and manner sufficient to eliminate the negative balance as quickly as possible.

An allocation of items of our income, gain, loss or deduction, other than an allocation required by the Internal Revenue Code to eliminate the difference between a partner's "book" capital account, credited with the fair market value of Contributed Property, and "tax" capital account, credited with the tax basis of Contributed Property, referred to in this discussion as the "Book-Tax Disparity," will generally be given effect for federal income tax purposes in determining a partner's share of an item of income, gain, loss or deduction only if the allocation has substantial economic effect. In any other case, a partner's share of an item will be determined on the basis of his interest in us, which will be determined by taking into account all the facts and circumstances, including:

•: his relative contributions to us;
•: the interests of all the partners in profits and losses;
•: the interest of all the partners in cash flow; and
•: the rights of all the partners to distributions of capital upon liquidation.

Vinson & Elkins L.L.P. is of the opinion that, with the exception of the issues described in "—Tax Consequences of Unit Ownership—Section 754 Election" and "—Disposition of Common Units—Allocations Between Transferors and Transferees," allocations under our partnership agreement will be given effect for federal income tax purposes in determining a unitholder's share of an item of income, gain, loss or deduction.

Treatment of Short Sales. A unitholder whose units are loaned to a "short seller" to cover a short sale of units may be considered as having disposed of those units. If so, he would no longer be treated for tax purposes as a partner with respect to those units during the period of the loan and may recognize gain or loss from the disposition. As a result, during this period:

•: any of our income, gain, loss or deduction with respect to those units would not be reportable by the unitholder;
•: any cash distributions received by the unitholder as to those units would be fully taxable; and
•: all of these distributions would appear to be ordinary income.

Vinson & Elkins L.L.P. has not rendered an opinion regarding the treatment of a unitholder where common units are loaned to a short seller to cover a short sale of common units; therefore, unitholders desiring to assure their status as partners and avoid the risk of gain recognition from a loan to a short

seller should modify any applicable brokerage account agreements to prohibit their brokers from borrowing their units. The IRS has announced that it is actively studying issues relating to the tax treatment of short sales of partnership interests. Please also read "—Disposition of Common Units—Recognition of Gain or Loss."

Alternative Minimum Tax. Each unitholder will be required to take into account his distributive share of any items of our income, gain, loss or deduction for purposes of the alternative minimum tax. The current minimum tax rate for noncorporate taxpayers is 26% on the first $175,000 of alternative minimum taxable income in excess of the exemption amount and 28% on any additional alternative minimum taxable income. Prospective unitholders are urged to consult with their tax advisors as to the impact of an investment in units on their liability for the alternative minimum tax.

Tax Rates. In general, the highest effective United States federal income tax rate for individuals currently is 35% and the maximum United States federal income tax rate for net capital gains of an individual currently is 15% if the asset disposed of was held for more than twelve months at the time of disposition.

Section 754 Election. We have made the election permitted by Section 754 of the Internal Revenue Code. That election is irrevocable without the consent of the IRS. The election will generally permit us to adjust a common unit purchaser's tax basis in our assets ("inside basis") under Section 743(b) of the Internal Revenue Code to reflect his purchase price. The Section 743(b) adjustment does not apply to a person who purchases common units directly from us and it belongs only to the purchaser and not to other unitholders. Please also read, however, "—Tax Consequences of Unit Ownership—Allocation of Income, Gain, Loss and Deduction." For purposes of this discussion, a unitholder's inside basis in our assets has two components: (1) his share of our tax basis in our assets ("common basis") and (2) his Section 743(b) adjustment to that basis.

Where the remedial allocation method is adopted (which we have adopted), the Treasury Regulations under Section 743 of the Internal Revenue Code require a portion of the Section 743(b) adjustment that is attributable to recovery property to be depreciated over the remaining cost recovery period for the Section 704(c) built-in gain. Under Treasury Regulation Section 1.167(c)-l(a)(6), a Section 743(b) adjustment attributable to property subject to depreciation under Section 167 of the Internal Revenue Code rather than cost recovery deductions under Section 168 is generally required to be depreciated using either the straight-line method or the 150% declining balance method. Under our partnership agreement, the general partner is authorized to take a position to preserve the uniformity of units even if that position is not consistent with these Treasury Regulations. Please read "—Tax Treatment of Operations—Uniformity of Units."

Although Vinson & Elkins L.L.P. is unable to opine as to the validity of this approach because there is no clear authority on this issue, we intend to depreciate the portion of a Section 743(b) adjustment attributable to unrealized appreciation in the value of Contributed Property, to the extent of any unamortized book-tax disparity, using a rate of depreciation or amortization derived from the depreciation or amortization method and useful life applied to the common basis of the property, or treat that portion as non-amortizable to the extent attributable to property the common basis of which is not amortizable. This method is consistent with the regulations under Section 743 of the Internal Revenue Code but is arguably inconsistent with Treasury Regulation Section 1.167(c)-1(a)(6), which is not expected to directly apply to a material portion of our assets. To the extent this Section 743(b) adjustment is attributable to appreciation in value in excess of the unamortized book-tax disparity, we will apply the rules described in the Treasury Regulations and legislative history. If we determine that this position cannot reasonably be taken, we may take a depreciation or amortization position under which all purchasers acquiring units in the same month would receive depreciation or amortization, whether attributable to common basis or a Section 743(b) adjustment, based upon the same applicable rate as if they had purchased a direct interest in our assets. This kind of aggregate approach may result

in lower annual depreciation or amortization deductions than would otherwise be allowable to some unitholders. Please read "—Tax Treatment of Operations—Uniformity of Units."

A Section 754 election is advantageous if the transferee's tax basis in his units is higher than the units' share of the aggregate tax basis of our assets immediately prior to the transfer. In that case, as a result of the election, the transferee would have, among other items, a greater amount of depreciation and depletion deductions and his share of any gain or loss on a sale of our assets would be less. Conversely, a Section 754 election is disadvantageous if the transferee's tax basis in his units is lower than those units' share of the aggregate tax basis of our assets immediately prior to the transfer. Thus, the fair market value of the units may be affected either favorably or unfavorably by the election. A basis adjustment is required regardless of whether a Section 754 election is made in the case of a transfer of an interest in us if we have a substantial built-in loss immediately after the transfer, or if we distribute property and have a substantial basis reduction. Generally a built-in loss or a basis reduction is substantial if it exceeds $250,000.

The calculations involved in the Section 754 election are complex and will be made on the basis of assumptions as to the value of our assets and other matters. For example, the allocation of the Section 743(b) adjustment among our assets must be made in accordance with the Internal Revenue Code. The IRS could seek to reallocate some or all of any Section 743(b) adjustment allocated by us to our tangible assets to goodwill instead. Goodwill, as an intangible asset, is generally amortizable over a longer period of time or under a less accelerated method than our tangible assets. We cannot assure you that the determinations we make will not be successfully challenged by the IRS and that the deductions resulting from them will not be reduced or disallowed altogether. Should the IRS require a different basis adjustment to be made, and should, in our opinion, the expense of compliance exceed the benefit of the election, we may seek permission from the IRS to revoke our Section 754 election. If permission is granted, a subsequent purchaser of units may be allocated more income than he would have been allocated had the election not been revoked.

Tax Treatment of Operations

Accounting Method and Taxable Year. We use the year ending December 31 as our taxable year and the accrual method of accounting for federal income tax purposes. Each unitholder will be required to include in income his share of our income, gain, loss and deduction for our taxable year ending within or with his taxable year. In addition, a unitholder who has a taxable year ending on a date other than December 31 and who disposes of all of his units following the close of our taxable year but before the close of his taxable year must include his share of our income, gain, loss and deduction in income for his taxable year, with the result that he will be required to include in income for his taxable year his share of more than one year of our income, gain, loss and deduction. Please read "—Disposition of Common Units—Allocations Between Transferors and Transferees."

Initial Tax Basis, Depreciation and Amortization. The tax basis of our assets will be used for purposes of computing depreciation and cost recovery deductions and, ultimately, gain or loss on the disposition of these assets. The federal income tax burden associated with the difference between the fair market value of our assets and their tax basis immediately prior to this offering will be borne by the general partner, its affiliates and our other unitholders. Please read "—Tax Consequences of Unit Ownership—Allocation of Income, Gain, Loss and Deduction."

To the extent allowable, we may elect to use the depreciation and cost recovery methods that will result in the largest deductions being taken in the early years after assets are placed in service. We were not entitled to any amortization deductions with respect to any goodwill conveyed to us on formation. Property we subsequently acquire or construct may be depreciated using accelerated methods permitted by the Internal Revenue Code.

If we dispose of depreciable property by sale, foreclosure, or otherwise, all or a portion of any gain, determined by reference to the amount of depreciation previously deducted and the nature of the property, may be subject to the recapture rules and taxed as ordinary income rather than capital gain. Similarly, a partner who has taken cost recovery or depreciation deductions with respect to property we own will likely be required to recapture some or all of those deductions as ordinary income upon a sale of his interest in us. Please read "—Tax Consequences of Unit Ownership—Allocation of Income, Gain, Loss and Deduction" and "—Disposition of Common Units—Recognition of Gain or Loss."

The costs incurred in selling our units (called "syndication expenses") must be capitalized and cannot be deducted currently, ratably or upon our termination. There are uncertainties regarding the classification of costs as organization expenses, which we may amortize, and as syndication expenses, which we may not amortize. The underwriting discounts and commissions we incur will be treated as syndication expenses.

Valuation and Tax Basis of Our Properties. The federal income tax consequences of the ownership and disposition of units will depend in part on our estimates of the relative fair market values, and the initial tax bases, of our assets. Although we may from time to time consult with professional appraisers regarding valuation matters, we will make many of the relative fair market value estimates ourselves. These estimates and determinations of basis are subject to challenge and will not be binding on the IRS or the courts. If the estimates of fair market value or basis are later found to be incorrect, the character and amount of items of income, gain, loss or deductions previously reported by unitholders might change, and unitholders might be required to adjust their tax liability for prior years and incur interest and penalties with respect to those adjustments.

Disposition of Common Units

Recognition of Gain or Loss. Gain or loss will be recognized on a sale of units equal to the difference between the amount realized and the unitholder's tax basis for the units sold. A unitholder's amount realized will be measured by the sum of the cash or the fair market value of other property he receives plus his share of our nonrecourse liabilities. Because the amount realized includes a unitholder's share of our nonrecourse liabilities, the gain recognized on the sale of units could result in a tax liability in excess of any cash received from the sale.

Prior distributions from us in excess of cumulative net taxable income for a common unit that decreased a unitholder's tax basis in that common unit will, in effect, become taxable income if the common unit is sold at a price greater than the unitholder's tax basis in that common unit, even if the price received is less than his original cost.

Except as noted below, gain or loss recognized by a unitholder, other than a "dealer" in units, on the sale or exchange of a unit held for more than one year will generally be taxable as capital gain or loss. Capital gain recognized by an individual on the sale of units held more than twelve months will generally be taxed at a maximum rate of 15%. A portion of this gain or loss, which will likely be substantial, however, will be separately computed and taxed as ordinary income or loss under Section 751 of the Internal Revenue Code to the extent attributable to assets giving rise to depreciation recapture or other "unrealized receivables" or to "inventory items" we own. The term "unrealized receivables" includes potential recapture items, including depreciation recapture. Ordinary income attributable to unrealized receivables, inventory items and depreciation recapture may exceed net taxable gain realized upon the sale of a unit and may be recognized even if there is a net taxable loss realized on the sale of a unit. Thus, a unitholder may recognize both ordinary income and a capital loss upon a sale of units. Net capital loss may offset capital gains and no more than $3,000 of ordinary income, in the case of individuals, and may only be used to offset capital gain in the case of corporations.

The IRS has ruled that a partner who acquires interests in a partnership in separate transactions must combine those interests and maintain a single adjusted tax basis for all those interests. Upon a sale or other disposition of less than all of those interests, a portion of that tax basis must be allocated to the interests sold using an "equitable apportionment" method. Treasury Regulations under Section 1223 of the Internal Revenue Code allow a selling unitholder who can identify common units transferred with an ascertainable holding period to elect to use the actual holding period of the common units transferred. Thus, according to the ruling, a common unitholder will be unable to select high or low basis common units to sell as would be the case with corporate stock, but, according to the regulations, may designate specific common units sold for purposes of determining the holding period of units transferred. A unitholder electing to use the actual holding period of common units transferred must consistently use that identification method for all subsequent sales or exchanges of common units. A unitholder considering the purchase of additional units or a sale of common units purchased in separate transactions is urged to consult his tax advisor as to the possible consequences of this ruling and application of the Treasury Regulations.

Specific provisions of the Internal Revenue Code affect the taxation of some financial products and securities, including partnership interests, by treating a taxpayer as having sold an "appreciated" partnership interest, one in which gain would be recognized if it were sold, assigned or terminated at its fair market value, if the taxpayer or related persons enter(s) into:

•: a short sale;
•: an offsetting notional principal contract; or
•: a futures or forward contract with respect to the partnership interest or substantially identical property.

Moreover, if a taxpayer has previously entered into a short sale, an offsetting notional principal contract or a futures or forward contract with respect to the partnership interest, the taxpayer will be treated as having sold that position if the taxpayer or a related person then acquires the partnership interest or substantially identical property. The Secretary of Treasury is also authorized to issue regulations that treat a taxpayer that enters into transactions or positions that have substantially the same effect as the preceding transactions as having constructively sold the financial position.

Allocations Between Transferors and Transferees. In general, our taxable income and losses will be determined annually, will be prorated on a monthly basis and will be subsequently apportioned among the unitholders in proportion to the number of units owned by each of them as of the opening of the applicable exchange on the first business day of the month (the "Allocation Date"). However, gain or loss realized on a sale or other disposition of our assets other than in the ordinary course of business will be allocated among the unitholders on the Allocation Date in the month in which that gain or loss is recognized. As a result, a unitholder transferring units may be allocated income, gain, loss and deduction realized after the date of transfer.

The use of this method may not be permitted under existing Treasury Regulations. Accordingly, Vinson & Elkins L.L.P. is unable to opine on the validity of this method of allocating income and deductions between unitholders. If this method is not allowed under the Treasury Regulations, or only applies to transfers of less than all of the unitholder's interest, our taxable income or losses might be reallocated among the unitholders. We are authorized to revise our method of allocation between unitholders to conform to a method permitted under future Treasury Regulations.

A unitholder who owns units at any time during a quarter and who disposes of them prior to the record date set for a cash distribution for that quarter will be allocated items of our income, gain, loss and deductions attributable to that quarter but will not be entitled to receive that cash distribution.

Notification Requirements. A unitholder who sells any of his units is generally required to notify us in writing of that sale within 30 days after the sale (or, if earlier, January 15 of the year following the sale). A purchaser of units who purchases units from another unitholder is also generally required to notify us in writing of that purchase within 30 days after the purchase. Upon receiving such notifications, we are required to notify the IRS of that transaction and to furnish specified information to the transferor and transferee. Failure to notify us of a purchase may, in some cases, lead to the imposition of substantial penalties. However, these reporting requirements do not apply to a sale by an individual who is a citizen of the United States and who effects the sale or exchange through a broker who will satisfy such requirements.

Constructive Termination. We will be considered to have been terminated for tax purposes if there is a sale or exchange of 50% or more of the total interests in our capital and profits within a twelve-month period. A constructive termination results in the closing of our taxable year for all unitholders. In the case of a unitholder reporting on a taxable year other than a fiscal year ending December 31, the closing of our taxable year may result in more than twelve months of our taxable income or loss being includable in his taxable income for the year of termination. We would be required to make new tax elections after a termination, including a new election under Section 754 of the Internal Revenue Code, and a termination would result in a deferral of our deductions for depreciation. A termination could also result in penalties if we were unable to determine that the termination had occurred. Moreover, a termination might either accelerate the application of, or subject us to, any tax legislation enacted before the termination.

Uniformity of Units

Because we cannot match transferors and transferees of units, we must maintain uniformity of the economic and tax characteristics of the units to a purchaser of these units. In the absence of uniformity, we may be unable to completely comply with a number of federal income tax requirements, both statutory and regulatory. A lack of uniformity can result from a literal application of Treasury Regulation Section 1.167(c)-1(a)(6). Any non-uniformity could have a negative impact on the value of the units. Please read "—Tax Consequences of Unit Ownership—Section 754 Election."

We intend to depreciate the portion of a Section 743(b) adjustment attributable to unrealized appreciation in the value of Contributed Property, to the extent of any unamortized book-tax disparity, using a rate of depreciation or amortization derived from the depreciation or amortization method and useful life applied to the common basis of that property, or treat that portion as nonamortizable, to the extent attributable to property the common basis of which is not amortizable, consistent with the regulations under Section 743 of the Internal Revenue Code, even though that position may be inconsistent with Treasury Regulation Section 1.167(c)-1(a)(6) which is not expected to directly apply to a material portion of our assets. Please read "—Tax Consequences of Unit Ownership—Section 754 Election." To the extent that the Section 743(b) adjustment is attributable to appreciation in value in excess of the unamortized book-tax disparity, we will apply the rules described in the Treasury Regulations and legislative history. If we determine that this position cannot reasonably be taken, we may adopt a depreciation and amortization position under which all purchasers acquiring units in the same month would receive depreciation and amortization deductions, whether attributable to a common basis or Section 743(b) adjustment, based upon the same applicable rate as if they had purchased a direct interest in our property. If this position is adopted, it may result in lower annual depreciation and amortization deductions than would otherwise be allowable to some unitholders and risk the loss of depreciation and amortization deductions not taken in the year that these deductions are otherwise allowable. This position will not be adopted if we determine that the loss of depreciation and amortization deductions will have a material adverse effect on the unitholders. If we choose not to utilize this aggregate method, we may use any other reasonable depreciation and amortization method to preserve the uniformity of the intrinsic tax characteristics of any units that would not have a material

adverse effect on the unitholders. The IRS may challenge any method of depreciating the Section 743(b) adjustment described in this paragraph. If this challenge were sustained, the uniformity of units might be affected, and the gain from the sale of units might be increased without the benefit of additional deductions. Please read "—Disposition of Common Units—Recognition of Gain or Loss."

Tax-Exempt Organizations and Other Investors

Ownership of units by employee benefit plans, other tax-exempt organizations, non-resident aliens, foreign corporations, and other foreign persons raises issues unique to those investors and, as described below, may have substantially adverse tax consequences to them.

Employee benefit plans and most other organizations exempt from federal income tax, including individual retirement accounts and other retirement plans, are subject to federal income tax on unrelated business taxable income. Virtually all of our income allocated to a unitholder which is a tax-exempt organization will be unrelated business taxable income and will be taxable to them.

Non-resident aliens and foreign corporations, trusts or estates that own units will be considered to be engaged in business in the United States because of the ownership of units. As a consequence they will be required to file federal tax returns to report their share of our income, gain, loss or deduction and pay federal income tax at regular rates on their share of our net income or gain. Under rules applicable to publicly traded partnerships, we will withhold tax, at the highest effective applicable rate, from cash distributions made quarterly to foreign unitholders. Each foreign unitholder must obtain a taxpayer identification number from the IRS and submit that number to our transfer agent on a Form W-8 or applicable substitute form in order to obtain credit for these withholding taxes. A change in applicable law may require us to change these procedures.

In addition, because a foreign corporation that owns units will be treated as engaged in a United States trade or business, that corporation may be subject to the United States branch profits tax at a rate of 30%, in addition to regular federal income tax, on its share of our income and gain, as adjusted for changes in the foreign corporation's "U.S. net equity," which is effectively connected with the conduct of a United States trade or business. That tax may be reduced or eliminated by an income tax treaty between the United States and the country in which the foreign corporate unitholder is a "qualified resident." In addition, this type of unitholder is subject to special information reporting requirements under Section 6038C of the Internal Revenue Code.

Under a ruling of the IRS, a foreign unitholder who sells or otherwise disposes of a unit will be subject to federal income tax on gain realized on the sale or disposition of that unit to the extent that this gain is effectively connected with a United States trade or business of the foreign unitholder. Apart from the ruling, a foreign unitholder will not be taxed or subject to withholding upon the sale or disposition of a unit if he has owned less than 5% in value of the units during the five-year period ending on the date of the disposition and if the units are regularly traded on an established securities market at the time of the sale or disposition.

Administrative Matters

Information Returns and Audit Procedures. We intend to furnish to each unitholder, within 90 days after the close of each calendar year, specific tax information, including a Schedule K-1, which describes his share of our income, gain, loss and deduction for our preceding taxable year. In preparing this information, which will not be reviewed by counsel, we will take various accounting and reporting positions, some of which have been mentioned earlier, to determine his share of income, gain, loss and deduction. We cannot assure you that those positions will yield a result that conforms to the requirements of the Internal Revenue Code, Treasury Regulations or administrative interpretations of the IRS. Neither we nor counsel can assure prospective unitholders that the IRS will not successfully

contend in court that those positions are impermissible. Any challenge by the IRS could negatively affect the value of the units.

The IRS may audit our federal income tax information returns. Adjustments resulting from an IRS audit may require each unitholder to adjust a prior year's tax liability, and possibly may result in an audit of his own return. Any audit of a unitholder's return could result in adjustments not related to our returns as well as those related to our returns.

Partnerships generally are treated as separate entities for purposes of federal tax audits, judicial review of administrative adjustments by the IRS and tax settlement proceedings. The tax treatment of partnership items of income, gain, loss and deduction are determined in a partnership proceeding rather than in separate proceedings with the partners. The Internal Revenue Code requires that one partner be designated as the "Tax Matters Partner" for these purposes. The partnership agreement names MarkWest Energy GP, L.L.C. as our Tax Matters Partner.

The Tax Matters Partner will make some elections on our behalf and on behalf of unitholders. In addition, the Tax Matters Partner can extend the statute of limitations for assessment of tax deficiencies against unitholders for items in our returns. The Tax Matters Partner may bind a unitholder with less than a 1% profits interest in us to a settlement with the IRS unless that unitholder elects, by filing a statement with the IRS, not to give that authority to the Tax Matters Partner. The Tax Matters Partner may seek judicial review, by which all the unitholders are bound, of a final partnership administrative adjustment and, if the Tax Matters Partner fails to seek judicial review, judicial review may be sought by any unitholder having at least a 1% interest in profits or by any group of unitholders having in the aggregate at least a 5% interest in profits. However, only one action for judicial review will go forward, and each unitholder with an interest in the outcome may participate.

A unitholder must file a statement with the IRS identifying the treatment of any item on his federal income tax return that is not consistent with the treatment of the item on our return. Intentional or negligent disregard of this consistency requirement may subject a unitholder to substantial penalties.

Nominee Reporting. Persons who hold an interest in us as a nominee for another person are required to furnish to us:

(a) the name, address and taxpayer identification number of the beneficial owner and the nominee;

(b) whether the beneficial owner is

(1) a person that is not a United States person,

(2) a foreign government, an international organization or any wholly owned agency or instrumentality of either of the foregoing, or

(3) a tax-exempt entity;

(d) specific information including the dates of acquisitions and transfers, means of acquisitions and transfers, and acquisition cost for purchases, as well as the amount of net proceeds from sales.

Brokers and financial institutions are required to furnish additional information, including whether they are United States persons and specific information on units they acquire, hold or transfer for their own account. A penalty of $50 per failure, up to a maximum of $100,000 per calendar year, is imposed

by the Internal Revenue Code for failure to report that information to us. The nominee is required to supply the beneficial owner of the units with the information furnished to us.

Accuracy-Related Penalties. An additional tax equal to 20% of the amount of any portion of an underpayment of tax that is attributable to one or more specified causes, including negligence or disregard of rules or regulations, substantial understatements of income tax and substantial valuation misstatements, is imposed by the Internal Revenue Code. No penalty will be imposed, however, for any portion of an underpayment if it is shown that there was a reasonable cause for that portion and that the taxpayer acted in good faith regarding that portion.

For individuals a substantial understatement of income tax in any taxable year exists if the amount of the understatement exceeds the greater of 10% of the tax required to be shown on the return for the taxable year or $5,000. The amount of any understatement subject to penalty generally is reduced if any portion is attributable to a position adopted on the return:

(1) for which there is, or was, "substantial authority," or

(2) as to which there is a reasonable basis and the relevant facts of that position are disclosed on the return.

If any item of income, gain, loss or deduction included in the distributive shares of unitholders might result in that kind of an "understatement" of income for which no "substantial authority" exists but for which a reasonable basis for the tax treatment of such item exists, we must disclose the relevant facts on our return. In such a case, we will make a reasonable effort to furnish sufficient information for unitholders to make adequate disclosure on their returns and to take other actions as may be appropriate to permit unitholders to avoid liability for this penalty. More stringent rules apply to "tax shelters," a term that in this context does not appear to include us.

A substantial valuation misstatement exists if the value of any property, or the adjusted basis of any property, claimed on a tax return is 200% or more of the amount determined to be the correct amount of the valuation or adjusted basis. No penalty is imposed unless the portion of the underpayment attributable to a substantial valuation misstatement exceeds $5,000 ($10,000 for most corporations). If the valuation claimed on a return is 400% or more than the correct valuation, the penalty imposed increases to 40%.

Reportable Transactions. If we were to engage in a "reportable transaction," we (and possibly you and others) would be required to make a detailed disclosure of the transaction to the IRS. A transaction may be a reportable transaction based upon any of several factors, including the fact that it is a type of tax avoidance transaction publicly identified by the IRS as a "listed transaction" or that it produces certain kinds of losses in excess of $2 million. Our participation in a reportable transaction could increase the likelihood that our federal income tax information return (and possibly your tax return) would be audited by the IRS. Please read "—Information Returns and Audit Procedures."

Moreover, if we were to participate in a reportable transaction with a significant purpose to avoid or evade tax, or in any listed transaction, you may be subject to the following provisions of the American Jobs Creation Act of 2004:

•: accuracy-related penalties with a broader scope, significantly narrower exceptions, and potentially greater amounts than described above at "—Accuracy-related Penalties,"
•: for those persons otherwise entitled to deduct interest on federal tax deficiencies, nondeductibility of interest on any resulting tax liability and
•: in the case of a listed transaction, an extended statute of limitations.

We do not expect to engage in any "reportable transactions."

Registration as a Tax Shelter. We registered as a "tax shelter" under the law in effect at the time of our initial public offering and were assigned tax shelter registration number 0218400024. Issuance of a tax shelter registration number to us does not indicate that investment in us or the claimed tax benefits have been reviewed, examined or approved by the IRS. The American Jobs Creation Act of 2004 (the "Jobs Act") repealed the tax shelter registration rules and replaced them with a new reporting regime. However, IRS Form 8271, as revised after the Jobs Act, nevertheless requires a unitholder to continue to report our tax shelter registration number on the unitholder's tax return for any year in which the unitholder claims any deduction, loss or other benefit, or reports any income, with respect to our common units. The IRS also appears to take the position that a unitholder who sells or transfers our common units after the Jobs Act must continue to provide our tax shelter registration number to the transferee. Unitholders are urged to consult their tax advisors regarding the application of the tax shelter registration rules.

State, Local and Other Tax Considerations

In addition to federal income taxes, you likely will be subject to other taxes, including state and local income taxes, unincorporated business taxes, and estate, inheritance or intangible taxes that may be imposed by the various jurisdictions in which we do business or own property or in which you are a resident. We currently do business or own property in nine states, most of which impose income taxes. We may also own property or do business in other states in the future. Although an analysis of those various taxes is not presented here, each prospective unitholder is urged to consider their potential impact on his investment in us. You may not be required to file a return and pay taxes in some states because your income from that state falls below the filing and payment requirement. You will be required, however, to file state income tax returns and to pay state income taxes in many of the states in which we do business or own property, and you may be subject to penalties for failure to comply with those requirements. In some states, tax losses may not produce a tax benefit in the year incurred and also may not be available to offset income in subsequent taxable years. Some of the states may require us, or we may elect, to withhold a percentage of income from amounts to be distributed to a unitholder who is not a resident of the state. Withholding, the amount of which may be greater or less than a particular unitholder's income tax liability to the state, generally does not relieve a non-resident unitholder from the obligation to file an income tax return. Amounts withheld may be treated as if distributed to unitholders for purposes of determining the amounts distributed by us. Please read "—Tax Consequences of Unit Ownership—Entity-Level Collections." Based on current law and our estimate of our future operations, the general partner anticipates that any amounts required to be withheld will not be material.

It is the responsibility of each unitholder to investigate the legal and tax consequences, under the laws of pertinent states and localities, of his investment in us. Vinson & Elkins L.L.P. has not rendered an opinion on the state or local tax consequences of an investment in us. We strongly recommend that each prospective unitholder consult, and depend upon, his own tax counsel or other advisor with regard to those matters. It is the responsibility of each unitholder to file all state and local, as well as United States federal tax returns, that may be required of him.

INVESTMENT IN MARKWEST ENERGY PARTNERS BY EMPLOYEE BENEFIT PLANS

An investment in us by an employee benefit plan is subject to additional considerations because the investments of these plans are subject to the fiduciary responsibility and prohibited transaction provisions of ERISA, and restrictions imposed by Section 4975 of the Internal Revenue Code. For these purposes the term "employee benefit plan" includes, but is not limited to, qualified pension, profit-sharing and stock bonus plans, Keogh plans, simplified employee pension plans and tax deferred annuities or IRAs established or maintained by an employer or employee organization. Among other things, consideration should be given to:

(a) whether the investment is prudent under Section 404(a)(1)(B) of ERISA;

(b) whether in making the investment, that plan will satisfy the diversification requirements of Section 404(a)(l)(C) of ERISA; and

(c) whether the investment will result in recognition of unrelated business taxable income by the plan and, if so, the potential after-tax investment return.

The person with investment discretion with respect to the assets of an employee benefit plan, often called a fiduciary, should determine whether an investment in us is authorized by the appropriate governing instrument and is a proper investment for the plan.

Section 406 of ERISA and Section 4975 of the Internal Revenue Code prohibit employee benefit plans, and IRAs that are not considered part of an employee benefit plan, from engaging in specified transactions involving "plan assets" with parties that are "parties in interest" under ERISA or "disqualified persons" under the Internal Revenue Code with respect to the plan.

In addition to considering whether the purchase of common units is a prohibited transaction, a fiduciary of an employee benefit plan should consider whether the plan will, by investing in us, be deemed to own an undivided interest in our assets, with the result that our general partner also would be fiduciaries of the plan and our operations would be subject to the regulatory restrictions of ERISA, including its prohibited transaction rules, as well as the prohibited transaction rules of the Internal Revenue Code.

The Department of Labor regulations provide guidance with respect to whether the assets of an entity in which employee benefit plans acquire equity interests would be deemed "plan assets" under some circumstances. Under these regulations, an entity's assets would not be considered to be "plan assets" if, among other things,

(a) the equity interests acquired by employee benefit plans are publicly offered securities; i.e., the equity interests are widely held by 100 or more investors independent of the issuer and each other, freely transferable and registered under some provisions of the federal securities laws,

(b) the entity is an "operating company," that is, it is primarily engaged in the production or sale of a product or service other than the investment of capital either directly or through a majority owned subsidiary or subsidiaries, or

(c) there is no significant investment by benefit plan investors, which is defined to mean that less than 25% of the value of each class of equity interest, disregarding some interests held by our general partner, its affiliates, and some other persons, is held by the employee benefit plans referred to above, IRAs and other employee benefit plans not subject to ERISA, including governmental plans.

Our assets should not be considered "plan assets" under these regulations because it is expected that the investment will satisfy the requirements in (a) above.

Plan fiduciaries contemplating a purchase of common units are urged to consult with their own counsel regarding the consequences under ERISA and the Internal Revenue Code in light of the serious penalties imposed on persons who engage in prohibited transactions or other violations.

DESCRIPTION OF COMMON UNITS

The Units

The common units and the subordinated units represent limited partner interests in us. The holders of units are entitled to participate in partnership distributions and exercise the rights or privileges available to limited partners under our partnership agreement. For a description of the relative rights and preferences of holders of common units and subordinated units in and to partnership distributions, please read "Cash Distribution Policy" and "Description of the Subordinated Units." For a description of the rights and privileges of limited partners under our partnership agreement, including voting rights, please read "The Partnership Agreement."

Transfer Agent and Registrar

Duties

Computershare Trust Company, Inc. serves as registrar and transfer agent for the common units. We pay all fees charged by the transfer agent for transfers of common units, except the following that must be paid by unitholders:

•: surety bond premiums to replace lost or stolen certificates, taxes and other governmental charges;
•: special charges for services requested by a holder of a common unit; and
•: other similar fees or charges.

There is no charge to unitholders for disbursements of our cash distributions. We will indemnify the transfer agent, its agents and each of their stockholders, directors, officers and employees against all claims and losses that may arise out of acts performed or omitted for its activities in that capacity, except for any liability due to any gross negligence or intentional misconduct of the indemnified person or entity.

Resignation or Removal

The transfer agent may resign, by notice to us, or be removed by us. The resignation or removal of the transfer agent will become effective upon our appointment of a successor transfer agent and registrar and its acceptance of the appointment. If no successor has been appointed and accepted the appointment within 30 days after notice of the resignation or removal, our general partner may act as the transfer agent and registrar until a successor is appointed.

Transfer of Common Units

The transfer of the common units to persons that purchase directly from the underwriters will be accomplished through the completion, execution and delivery of a transfer application by the investor. Any later transfers of a common unit will not be recorded by the transfer agent or recognized by us unless the transferee executes and delivers a transfer application. By executing and delivering a transfer application, the transferee of common units:

•: becomes the record holder of the common units and is an assignee until admitted into our partnership as a substituted limited partner;
•: automatically requests admission as a substituted limited partner in our partnership;
•: agrees to be bound by the terms and conditions of, and executes, our partnership agreement;
•: represents that the transferee has the capacity, power and authority to enter into the partnership agreement;

•: grants powers of attorney to officers of our general partner and any liquidator of us as specified in the partnership agreement; and
•: makes the consents and waivers contained in the partnership agreement.

An assignee will become a substituted limited partner of our partnership for the transferred common units upon the consent of our general partner and the recording of the name of the assignee on our books and records. Our general partner may withhold its consent in its sole discretion.

A transferee's broker, agent or nominee may complete, execute and deliver a transfer application. We are entitled to treat the nominee holder of a common unit as the absolute owner. In that case, the beneficial holder's rights are limited solely to those that it has against the nominee holder as a result of any agreement between the beneficial owner and the nominee holder.

Common units are securities and are transferable according to the laws governing transfer of securities. In addition to other rights acquired upon transfer, the transferor gives the transferee the right to request admission as a substituted limited partner in our partnership for the transferred common units. A purchaser or transferee of common units who does not execute and deliver a transfer application obtains only:

•: the right to assign the common unit to a purchaser or other transferee; and
•: the right to transfer the right to seek admission as a substituted limited partner in our partnership for the transferred common units.

Thus, a purchaser or transferee of common units who does not execute and deliver a transfer application:

•: will not receive cash distributions or federal income tax allocations, unless the common units are held in a nominee or "street name" account and the nominee or broker has executed and delivered a transfer application; and
•: may not receive some federal income tax information or reports furnished to record holders of common units.

The transferor of common units has a duty to provide the transferee with all information that may be necessary to transfer the common units. The transferor does not have a duty to insure the execution of the transfer application by the transferee and has no liability or responsibility if the transferee neglects or chooses not to execute and forward the transfer application to the transfer agent. Please read "The Partnership Agreement—Status as Limited Partner or Assignee."

Until a common unit has been transferred on our books, we and the transfer agent may treat the record holder of the unit as the absolute owner for all purposes, except as otherwise required by law or applicable stock exchange regulations.

PLAN OF DISTRIBUTION

We are registering the common units on behalf of the selling unitholders. As used in this prospectus, "selling unitholders" includes donees and pledgees selling common units received from a named selling unitholder after the date of this prospectus.

Under this prospectus, the selling unitholders intend to offer our securities to the public:

•: through one or more broker-dealers;
•: through underwriters; or
•: directly to investors.

The selling unitholders may price the common units offered from time to time:

•: at fixed prices;
•: at market prices prevailing at the time of any sale under this registration statement;
•: at prices related to prevailing market prices;
•: varying prices determined at the time of sale; or
•: at negotiated prices.

We will pay the costs and expenses of the registration and offering of the common units offered hereby. We will not pay any underwriting fees, discounts and selling commissions allocable to each selling unitholder's sale of its respective common units, which will be paid by the selling unitholders. Broker-dealers may act as agent or may purchase securities as principal and thereafter resell the securities from time to time:

•: in or through one or more transactions (which may involve crosses and block transactions) or distributions;
•: on the American Stock Exchange;
•: through the writing of options;
•: in the over-the-counter market; or
•: in private transactions.

Broker-dealers or underwriters may receive compensation in the form of underwriting discounts or commissions and may receive commissions from purchasers of the securities for whom they may act as agents. If any broker-dealer purchases the securities as principal, it may effect resales of the securities from time to time to or through other broker-dealers, and other broker-dealers may receive compensation in the form of concessions or commissions from the purchasers of securities for whom they may act as agents.

To the extent required, the names of the specific managing underwriter or underwriters, if any, as well as other important information, will be set forth in prospectus supplements. In that event, the discounts and commissions the selling unitholders will allow or pay to the underwriters, if any, and the discounts and commissions the underwriters may allow or pay to dealers or agents, if any, will be set forth in, or may be calculated from, the prospectus supplements. Any underwriters, brokers, dealers and agents who participate in any sale of the securities may also engage in transactions with, or perform services for, us or our affiliates in the ordinary course of their businesses.

In addition, the selling unitholders have advised us that they may sell common units in compliance with Rule 144, if available, or pursuant to other available exemptions from the registration requirements under the Securities Act, rather than pursuant to this prospectus.

To the extent required, this prospectus may be amended or supplemented from time to time to describe a specific plan of distribution.

In connection with offerings under this shelf registration and in compliance with applicable law, underwriters, brokers or dealers may engage in transactions which stabilize or maintain the market price of the securities at levels above those which might otherwise prevail in the open market. Specifically, underwriters, brokers or dealers may over-allot in connection with offerings, creating a short position in the securities for their own accounts. For the purpose of covering a syndicate short position or stabilizing the price of the securities, the underwriters, brokers or dealers may place bids for the securities or effect purchases of the securities in the open market. Finally, the underwriters may impose a penalty whereby selling concessions allowed to syndicate members or other brokers or dealers for distribution the securities in offerings may be reclaimed by the syndicate if the syndicate repurchases previously distributed securities in transactions to cover short positions, in stabilization transactions or otherwise. These activities may stabilize, maintain or otherwise affect the market price of the securities, which may be higher than the price that might otherwise prevail in the open market, and, if commenced, may be discontinued at any time.

VALIDITY OF THE COMMON UNITS

The validity of the common units will be passed upon for us by Vinson & Elkins L.L.P., Houston, Texas.

EXPERTS

The consolidated financial statements as of and for the year ended December 31, 2005 and management's report on the effectiveness of internal control over financial reporting as of December 31, 2005 incorporated in this prospectus by reference from MarkWest Energy Partners, L.P.'s Amendment No. 2 to the Annual Report on Form 10-K/A for the year ended December 31, 2005 have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports (which reports (1) express an unqualified opinion on the financial statements, (2) express an unqualified opinion on management's assessment regarding the effectiveness of internal control over financial reporting, and (3) express an adverse opinion on the effectiveness of internal control over financial reporting), which are incorporated herein by reference, and have been so incorporated in reliance upon the reports of such firm given upon their authority as experts in accounting and auditing.

The consolidated financial statements of MarkWest Energy Partners, L.P. as of December 31, 2004 and for the year then ended have been incorporated by reference herein and in the registration statement in reliance upon the report of KPMG LLP, independent registered public accounting firm, incorporated by reference herein, and upon the authority of said firm as experts in accounting and auditing.

The consolidated statements of operations, of comprehensive income, of changes in capital and cash flows for the year ended December 31, 2003 incorporated in this Prospectus by reference to Amendment No. 2 to the Annual Report on Form 10-K/A for the year ended December 31, 2005 have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.

The financial statements of Javelina Company as of December 31, 2004 and 2003 and for each of the three years in the period ended December 31, 2004 incorporated by reference into this Prospectus by reference to the current report on Form 8-K/A of MarkWest Energy Partners L.P. filed December 22, 2005, have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given the authority of said firm as experts in auditing and accounting.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission. We have also filed with the SEC under the Securities Act a registration statement on Form S-3 with respect to the common units offered by this prospectus. This prospectus, which constitutes part of the registration statement, does not contain all the information set forth in the registration statement or the exhibits and schedules which are part of the registration statement, portions of which are omitted as permitted by the rules and regulations of the SEC. Statements made in this prospectus regarding the contents of any contract or other document are summaries of the material terms of the contract or document. With respect to each contract or document filed as an exhibit to the registration statement, reference is made to the corresponding exhibit. For further information pertaining to us and the common units offered by this prospectus, reference is made to the registration statement, including the exhibits and schedules thereto, copies of which may be inspected without charge at the public reference facilities of the SEC at 100 F Street, N.E., Washington, D.C. 20549. Copies of all or any portion of the registration statement may be obtained from the SEC at prescribed rates. Information on the public reference facilities may be obtained by calling the SEC at 1-800-SEC-0330. In addition, the SEC maintains a web site that contains reports, proxy and information statements and other information that is filed through the SEC's EDGAR System. The web site can be accessed at http://www.sec.gov.

The SEC allows us to "incorporate by reference" the information we have filed with it, which means that we can disclose important information to you by referring you to those documents. The information we incorporate by reference is an important part of this prospectus. We incorporate by reference the documents listed below and any future filings we make with the SEC under Section 13(a), 13(f), 14 or 15(d) of the Securities Exchange Act of 1934 (excluding any information furnished pursuant to 2.02 or 7.01 on any Current Report on Form 8-K) until all offerings under this registration statement are completed:

•: our Annual Report on Form 10-K for the year ended December 31, 2005, as filed with the SEC on March 16, 2006 as amended by Amendment No. 1 and Amendment No. 2 to our Annual Report on Form 10-K/A for the year ended December 31, 2005, as filed with the SEC on March 31, 2006 and June 20, 2006, respectively;
•: our Quarterly Report on Form 10-Q for the three months ended March 31, 2006 filed with the SEC May 3, 2006 as amended by Amendment No. 1 and Amendment No. 2 to our Quarterly Report on Form 10-Q/A for the three months ended March 31, 2006, as filed with the SEC on May 4, 2006 and June 20, 2006, respectively, our Quarterly Report on Form 10-Q for the three months ended June 30, 2006 as filed with the SEC on August 4, 2006 and our Quarterly Report on Form 10-Q for the three months ended September 30, 2006 as filed with the SEC on November 7, 2006; and
•: our current reports on Form 8-K as filed with the SEC on January 5, 2006, January 26, 2006, February 2, 2006, March 10, 2006 April 25, 2006, April 27, 2006, May 4, 2006, May 12, 2006, July 5, 2006, July 7, 2006 July 28, 2006, September 7, 2006, September 26, 2006, October 24, 2006, October 27, 2006, October 30, 2006 and November 1, 2006 and Amendment No. 1 to our current report on Form 8-K/A as filed with the SEC on December 22, 2005.

We will provide to each person, including any beneficial owner, to whom a prospectus is delivered a copy of these filings, other than an exhibit to these filings unless we have specifically incorporated that exhibit by reference into the filing, upon written or oral request and at no cost. Requests should be made by writing or telephoning us at the following address:

MarkWest Energy Partners, L.P.
1515 Arapahoe Street
Denver, Colorado 80202
(303) 925-9200
Attention: Investor Relations

These documents may also be accessed at our website at http://www.markwestenergy.com. All other information contained in or accessible from our corporate website is not part of this prospectus.

FORWARD-LOOKING STATEMENTS

Statements included or incorporated by reference in this prospectus that are not historical facts are forward-looking statements. We use words such as "may," "believe," "estimate," "expect," "intend," "project," "anticipate," and similar expressions to identify forward-looking statements.

These forward-looking statements are made based upon management's expectations, estimates, assumptions and beliefs concerning future events impacting us and therefore involve a number of risks and uncertainties. We caution that forward-looking statements are not guarantees and that actual results could differ materially from those expressed or implied in the forward-looking statements.

Important factors that could cause our actual results of operations or our actual financial condition to differ include, but are not necessarily limited to:

•: our ability to successfully integrate our recent or future acquisitions;
•: the availability of natural gas supply for our gathering and processing services;
•: the availability of crude oil refinery runs to feed our Javelina off-gas processing facility;
•: the availability of NGLs for our transportation, fractionation and storage services;
•: our dependence on certain significant customers, producers, gatherers, treaters, and transporters of natural gas, including MarkWest Hydrocarbon;
•: the risks that third-party oil and gas exploration and production activities will not occur or be successful;
•: prices of natural gas and NGL products, including the effectiveness of any hedging activities;
•: competition from other NGL processors, including major energy companies;
•: changes in general economic, market or business conditions in regions where our products are located;
•: our ability to identify and complete organic growth projects or acquisitions complementary to our business;
•: the success of our risk management policies;
•: continued creditworthiness of, and performance by, contract counterparties;
•: operational hazards and availability and cost of insurance on our assets and operations;
•: the impact of any failure of our information technology systems;
•: the impact of current and future laws and government regulations;
•: liability for environmental claims;
•: damage to facilities and interruption of service due to casualty, weather, mechanical failure or any extended or extraordinary maintenance or inspection that may be required;
•: the impact of the departure of any key executive officers; and
•: our ability to raise sufficient capital to execute our business plan through borrowing or issuing equity.

This list is not necessarily complete. Other unknown or unpredictable factors could also have material adverse effects on future results. We do not update publicly any forward-looking statement with new information or future events. Investors are cautioned not to put undue reliance on forward-looking statements as many of these factors are beyond our ability to control or predict. Please read "Risk Factors."

GLOSSARY OF TERMS

We have included below the definitions for certain terms used in this prospectus:

adjusted operating surplus: For any period, operating surplus generated during that period is adjusted to:

(a) decrease operating surplus by:

(1) any net increase in working capital borrowings during that period; and

(2) any net reduction in cash reserves for operating expenditures during that period not relating to an operating expenditure made during that period; and

(b) increase operating surplus by:

(1) any net decrease in working capital borrowings during that period; and

(2) any net increase in cash reserves for operating expenditures during that period required by any debt instrument for the repayment of principal, interest or premium.

Adjusted operating surplus does not include that portion of operating surplus included in clause (a) (1) of the definition of operating surplus.

available cash: For any quarter ending prior to liquidation:

(a) the sum of:

(1) all cash and cash equivalents of MarkWest Energy Partners, L.P. and its subsidiaries on hand at the end of that quarter; and

(2) all additional cash and cash equivalents of MarkWest Energy Partners, L.P. and its subsidiaries on hand on the date of determination of available cash for that quarter resulting from working capital borrowings made after the end of that quarter;

(b) less the amount of cash reserves that is necessary or appropriate in the reasonable discretion of our general partner to:

(1) provide for the proper conduct of the business of MarkWest Energy Partners, L.P. and its subsidiaries (including reserves for future capital expenditures and for future credit needs of MarkWest Energy Partners, L.P. and its subsidiaries) after that quarter;

(2) comply with applicable law or any debt instrument or other agreement or obligation to which MarkWest Energy Partners, L.P. or any of its subsidiaries is a party or its assets are subject; and

(3) provide funds for minimum quarterly distributions and cumulative common unit arrearages for any one or more of the next four quarters;

provided, however, that our general partner may not establish cash reserves for distributions to the subordinated units unless our general partner has determined that in its judgment the establishment of reserves will not prevent MarkWest Energy Partners, L.P. from distributing the minimum quarterly distribution on all common units and any cumulative common unit arrearages thereon for the next four quarters; and provided, further, that disbursements made by MarkWest Energy Partners, L.P. or any of its subsidiaries or cash reserves established, increased or reduced after the end of that quarter but on or before the date of determination of available cash for that quarter shall be deemed to have been made, established, increased or reduced, for purposes of determining available cash, within that quarter if our general partner so determines.

barrel: One barrel of petroleum products equals 42 U.S. gallons.

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Bcf: One billion cubic feet of natural gas.

bpd: Barrels per day.

btu: British Thermal Units.

capital account: The capital account maintained for a partner under the partnership agreement. The capital account of a partner for a common unit, a subordinated unit, an incentive distribution right or any other partnership interest will be the amount which that capital account would be if that common unit, subordinated unit, incentive distribution right or other partnership interest were the only interest in MarkWest Energy Partners, L.P. held by a partner.

capital surplus: All available cash distributed by us from any source will be treated as distributed from operating surplus until the sum of all available cash distributed since the closing of the initial public offering equals the operating surplus as of the end of the quarter before that distribution. Any excess available cash will be deemed to be capital surplus.

closing price: The last sale price on a day, regular way, or in case no sale takes place on that day, the average of the closing bid and asked prices on that day, regular way. In either case, as reported in the principal consolidated transaction reporting system for securities listed or admitted to trading on the principal national securities exchange on which the units of that class are listed or admitted to trading. If the units of that class are not listed or admitted to trading on any national securities exchange, the last quoted price on that day. If no quoted price exists, the average of the high bid and low asked prices on that day in the over-the-counter market, as reported by the Nasdaq Stock Market or any other system then in use. If on any day the units of that class are not quoted by any organization of that type, the average of the closing bid and asked prices on that day as furnished by a professional market maker making a market in the units of the class selected by our general partner. If on that day no market maker is making a market in the units of that class, the fair value of the units on that day as determined reasonably and in good faith by our general partner.

common unit arrearage: The amount by which the minimum quarterly distribution for a quarter during the subordination period exceeds the distribution of available cash from operating surplus actually made for that quarter on a common unit, cumulative for that quarter and all prior quarters during the subordination period.

current market price: For any class of units listed or admitted to trading on any national securities exchange as of any date, the average of the daily closing prices for the 20 consecutive trading days immediately prior to that date.

incentive distribution right: A non-voting limited partner partnership interest issued to our general partner in connection with the formation of the Partnership. The partnership interest will confer upon its holder only the rights and obligations specifically provided in the partnership agreement for incentive distribution rights.

incentive distributions: The distributions of available cash from operating surplus initially made to our general partner that are in excess of our general partner's aggregate 2% general partner interest.

interim capital transactions: The following transactions if they occur prior to liquidation:

(a) borrowings, refinancings or refundings of indebtedness and sales of debt securities (other than for working capital borrowings and other than for items purchased on open account in the ordinary course of business) by MarkWest Energy Partners, L.P. or any of its subsidiaries;

(b) sales of equity interests by MarkWest Energy Partners, L.P. or any of its subsidiaries;

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(c) sales or other voluntary or involuntary dispositions of any assets of MarkWest Energy Partners, L.P. or any of its subsidiaries (other than sales or other dispositions of inventory, accounts receivable and other assets in the ordinary course of business, and sales or other dispositions of assets as a part of normal retirements or replacements).

Mcf: One thousand cubic feet.

Mcfe: One thousand cubic feet equivalent, determined using the ratio of six Mcf of natural gas to one barrel of crude oil, condensate or natural gas liquids, which approximates the relative energy content of crude oil, condensate and natural gas liquids as compared to natural gas.

Mcf/d: One thousand cubic feet per day.

Mcfe/d: One thousand cubic feet equivalent per day.

MMBtu/d: One million British Thermal Units per day.

NGLs: natural gas liquids, including propane, butane, isobutane, normal butane and natural gasoline.

operating expenditures: All expenditures of MarkWest Energy Partners, L.P. and our subsidiaries, including, but not limited to, facility operating costs, taxes, reimbursements of our general partner, repayment of working capital borrowings, debt service payments and capital expenditures, subject to the following:

(a) Payments (including prepayments) of principal of and premium on indebtedness, other than working capital borrowings will not constitute operating expenditures.

(b) Operating expenditures will not include:

(1) capital expenditures made for acquisitions or for capital improvements;

(2) payment of transaction expenses relating to interim capital transactions; or

(3) distributions to partners.

operating surplus: For any period prior to liquidation, on a cumulative basis and without duplication:

(a) the sum of

(1) $6.3 million plus all the cash of MarkWest Energy Partners, L.P. and its subsidiaries on hand as of the closing date of our initial public offering;

(2) all cash receipts of MarkWest Energy Partners, L.P. and our subsidiaries for the period beginning on the closing date of our initial public offering and ending with the last day of that period, other than cash receipts from interim capital transactions; and

(3) all cash receipts of MarkWest Energy Partners, L.P. and our subsidiaries after the end of that period but on or before the date of determination of operating surplus for the period resulting from working capital borrowings; less

(b) the sum of:

(1) operating expenditures for the period beginning on the closing date of our initial public offering and ending with the last day of that period; and

(2) the amount of cash reserves that is necessary or advisable in the reasonable discretion of our general partner to provide funds for future operating expenditures; provided however, that disbursements made (including contributions to a member of MarkWest Energy

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Partners, L.P. and our subsidiaries or disbursements on behalf of a member of MarkWest Energy Partners, L.P. and our subsidiaries) or cash reserves established, increased or reduced after the end of that period but on or before the date of determination of available cash for that period shall be deemed to have been made, established, increased or reduced for purposes of determining operating surplus, within that period if our general partner so determines.

subordination period: The subordination period will generally extend from the closing of the initial public offering until the first to occur of:

(a) the first day of any quarter beginning after June 30, 2009, for which:

(1) distributions of available cash from operating surplus on each of the outstanding common units and subordinated units equaled or exceeded the sum of the minimum quarterly distribution on all of the outstanding common units and subordinated units for each of the three consecutive, non-overlapping four-quarter periods immediately preceding that date;

(2) the adjusted operating surplus generated during each of the three consecutive, non-overlapping four-quarter periods immediately preceding that date equaled or exceeded the sum of the minimum quarterly distribution on all of the common units and subordinated units that were outstanding during those periods on a fully-diluted basis, and the related distribution on our general partner interest in MarkWest Energy Partners, L.P.; and

(3)there are no outstanding cumulative common units arrearages.

(b) the date on which our general partner is removed as general partner of MarkWest Energy Partners, L.P. upon the requisite vote by the limited partners under circumstances where cause does not exist and units held by our general partner and its affiliates are not voted in favor of the removal.

provided, however, subordinated units may convert into common units as described in "Cash Distribution Policy—Subordination Period—Early Conversion of Subordinated Units."

throughput: The volume of gas transported or passing through a pipeline, plant or other facility.

working capital borrowings: Borrowings exclusively for working capital purposes made pursuant to a credit facility or other arrangement requiring all borrowings thereunder to be reduced to a relatively small amount each year for an economically meaningful period of time.

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PART II: INFORMATION NOT REQUIRED IN THE PROSPECTUS

Item 13. Other Expenses of Issuance and Distribution.

The following is a list of estimated expenses in connection with the issuance and distribution of the securities being registered, with the exception of underwriting discounts and commissions:

SEC registration fee		$	18,005.45
Printing costs			15,000
Legal fees and expenses			60,000
Accounting fees and expenses			75,000
Miscellaneous			5,000

	Total	$	173,005.45

All of the above expenses except the SEC registration fee are estimates. All of the above expenses will be borne by the Registrant.

Item 14. Indemnification of Directors and Officers.

•: our general partner;
•: any departing general partner;
•: any person who is or was an affiliate of a general partner or any departing general partner;
•: any person who is or was a member, partner, officer, director, employee, agent or trustee of our general partner or any departing general partner or any affiliate of a general partner or any departing general partner; or
•: any person who is or was serving at the request of a general partner or any departing general partner or any affiliate of a general partner or any departing general partner as an officer, director, employee, member, partner, agent or trustee of another person.

Subject to any terms, conditions or restrictions set forth in the Partnership Agreement, Section 17-108 of the Delaware Revised Uniform Limited Partnership Act empowers a Delaware limited partnership to indemnify and hold harmless any partner or other person from and against all claims and demands whatsoever.

Item 15. Recent Sales of Unregistered Securities.

In June 2003, MarkWest Energy Partners, L.P. sold 375,000 common units to certain accredited investors in a private placement. Gross proceeds of the sale were approximately $9.8 million. The proceeds were used to reduce outstanding indebtedness. In connection with the sale, MarkWest Energy Partners, L.P. granted demand and piggyback registration rights to the purchasers, pursuant to a registration rights agreement. The issuance of these securities were made in reliance upon the exemption from registration requirements provided by Section 4(2) of the Securities Act of 1933, as amended, for transactions by an issuer not involving a public offering.

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In July 2004, MarkWest Energy Partners, L.P. sold 1,304,438 common units to certain accredited investors in a private placement. Gross proceeds of the sale were approximately $43.9 million. The proceeds were used to reduce outstanding indebtedness. In connection with the sale, MarkWest Energy Partners, L.P. granted shelf and piggyback registration rights to the purchasers, pursuant to a registration rights agreement. The issuance of these securities were made in reliance upon the exemption from registration requirements provided by Section 4(2) of the Securities Act of 1933, as amended, for transactions by an issuer not involving a public offering.

In October 2004, MarkWest Energy Partners, L.P. issued $225.0 million aggregate principal amount of its 6.875% senior notes due 2014 to J.P Morgan Inc. and other initial purchasers of the senior notes. The Registrant issued the senior notes to the initial purchasers in reliance on Section 4(2) of the Securities Act on the basis that each initial purchaser represented and warranted to the Registrant that it was (i) a qualified institutional buyer as defined in Rule 144A under the Securities Act and (ii) an "accredited investor" within the meaning of Rule 501(a) under the Securities Act. The initial purchasers then offered and resold the notes to qualified institutional buyers in reliance on Rule 144A under the Securities Act and to non-U.S. persons in transactions outside the United States in reliance on Regulation S under the Securities Act. The Registrant exchanged the senior notes for new registered senior notes with substantially identical terms in March 2006.

In November 2005, MarkWest Energy Partners, L.P. sold 1,644,065 common units to certain accredited investors in a private placement. Gross proceeds of the sale were approximately $72.7 million. The proceeds were used to reduce outstanding indebtedness. In connection with the sale, MarkWest Energy Partners, L.P. granted shelf and piggyback registration rights to the purchasers, pursuant to a registration rights agreement. The issuance of these securities were made in reliance upon the exemption from registration requirements provided by Section 4(2) of the Securities Act of 1933, as amended, for transactions by an issuer not involving a public offering.

In December 2005, MarkWest Energy Partners, L.P. sold 574,714 common units to certain accredited investors in a private placement. Gross proceeds of the sale were approximately $25.0 million. The proceeds were used to reduce outstanding indebtedness. In connection with the sale, MarkWest Energy Partners, L.P. granted shelf and piggyback registration rights to the purchasers, pursuant to a registration rights agreement. The issuance of these securities were made in reliance upon the exemption from registration requirements provided by Section 4(2) of the Securities Act of 1933, as amended, for transactions by an issuer not involving a public offering.

In June 2006, the Registrant issued $200.0 million aggregate principal amount of its 8¹/2% senior notes due 2016 to RBC Capital Markets Corporation, J.P. Morgan Securities Inc., Wachovia Capital Markets, LLC and other initial purchasers of the senior notes. The Registrant used the net proceeds of the offering to repay a portion of borrowings outstanding under its credit facility incurred in connection with recent acquisitions. The Registrant issued the senior notes to the initial purchasers in reliance on Section 4(2) of the Securities Act on the basis that each initial purchaser represented and warranted to the Registrant that it was (i) a qualified institutional buyer as defined in Rule 144A under the Securities Act and (ii) an "accredited investor" within the meaning of Rule 501(a) under the Securities Act. The initial purchasers then offered and resold the notes to qualified institutional buyers in reliance on Rule 144A under the Securities Act.

In October 2006, the Registrant issued $75.0 million aggregate principal amount of its 8¹/2% senior notes due 2016 to RBC Capital Markets Corporation and other initial purchasers of the senior notes. The Registrant used the net proceeds of the offering to repay a portion of borrowings outstanding under its credit facility incurred in connection with recent acquisitions. The Registrant issued the senior notes to the initial purchasers in reliance on Section 4(2) of the Securities Act on the basis that each initial purchaser represented and warranted to the Registrant that it was (i) a qualified institutional buyer as defined in Rule 144A under the Securities Act and (ii) an "accredited investor" within the

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meaning of Rule 501(a) under the Securities Act. The initial purchasers then offered and resold the notes to qualified institutional buyers in reliance on Rule 144A under the Securities Act.

Item 16. Exhibits and Financial Statement Schedules.

(a) Exhibits. The exhibits listed in the accompanying Exhibit Index are filed (except where otherwise indicated) as part of this Registration Statement.

(b) Financial Statement Schedules. We incorporate by reference into this prospectus (i) our historical financial statements and the accompanying notes in our Annual Report on Form 10-K filed with the SEC on March 16, 2006, (ii) the historical financial statements of Javelina and the accompanying notes in our Current Report on Form 8-K/A filed with the SEC on December 22, 2005 and (iii) the historical financial statements of Starfish and the accompanying notes in Amendment No. 1 to our Annual Report on Form 10-K/A filed with the SEC on March 31, 2006.

All schedules are omitted since the required information is not present, or is not present in amounts sufficient to require submission of the schedule, or because the information required is included in the consolidated financial statements and notes thereto.

Item 17. Undertakings.

The undersigned registrant hereby undertakes to file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement:

(i) To include any prospectus required by Section 10(a)(3) of the Securities Act of 1933 (the "Securities Act");

(ii) To reflect in the prospectus any facts or events arising after the effective date of this registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in this registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of a prospectus filed with the Securities and Exchange Commission (the "SEC") pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than a 20% change in the maximum aggregate offering price set forth in the "Calculation of Registration Fee" table in the effective registration statement; and

(iii) To include any material information with respect to the plan of distribution not previously disclosed in this registration statement or any material change to such information in this registration statement;

provided, however, that paragraphs (1)(i) and (1)(ii) above do not apply if information required to be included in a post-effective amendment by those paragraphs is contained in periodic reports filed with or furnished to the SEC by the registrant pursuant to Section 13 or Section 15(d) of the Exchange Act of 1934 (the "Exchange Act") that are incorporated by reference in the registration statement or is contained in a form of prospectus filed pursuant to Rule 424(b) that is part of the registration statement;

The undersigned registrant hereby undertakes, for the purpose of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

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The undersigned registrant hereby undertakes remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

The undersigned registrant hereby undertakes to provide to the underwriters at the closing specified in the underwriting agreement certificates in such denominations and registered in such names as required by the underwriters to permit prompt delivery to each purchaser.

Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

The registrant hereby undertakes that, for purposes of determining any liability under the Securities Act of 1933, each filing of the registrant's annual report pursuant to Section 13(a) or Section 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing of an employee benefit plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of 1934) that is incorporated by reference in this registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

The undersigned registrant hereby undertakes that:

(1): For purposes of determining any liability under the Securities Act, the information omitted from the form of prospectus filed as part of this registration statement in reliance upon Rule 430A and contained in a form of prospectus filed by the registrant pursuant to Rule 424(b)(1) or (4) or 497(h) under the Securities Act shall be deemed to be part of this registration statement as of the time it was declared effective.
(2): For the purpose of determining any liability under the Securities Act, each post-effective amendment that contains a form of prospectus shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Denver, state of Colorado, on December 6, 2006.

	MARKWEST ENERGY PARTNERS, L.P
		(Registrant)
	By:	MARKWEST ENERGY GP, L.L.C., Its General Partner
Date: December 6, 2006	By:	*
		Frank M. Semple President and Chief Executive Officer (Principal Executive Officer)

Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.

Date: December 6, 2006	By:	*
		Frank M. Semple President and Chief Executive Officer (Principal Executive Officer)
Date: December 6, 2006	By:	*
		Nancy K. Buese Chief Financial Officer (Principal Financial Officer and Principal Accounting Officer)
Date: December 6, 2006	By:	*
		John M. Fox Chairman of the Board and Director
Date: December 6, 2006	By:	*
		Keith E. Bailey Director
Date: December 6, 2006	By:	*
		Charles K. Dempster Director
Date: December 6, 2006	By:	*
		Donald C. Heppermann Director

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Date: December 6, 2006	By:	*
		William A. Kellstrom Director
Date: December 6, 2006	By:	*
		William P. Nicoletti Director

*By:

/s/ ANDREW L. SCHROEDER

Andrew L. Schroeder
Attorney-in-Fact

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EXHIBITS AND FINANCIAL STATEMENT SCHEDULES

(a)

The following documents are filed as part of this report:

(1): Financial Statements:

We incorporate by reference into this prospectus (i) our historical financial statements and the accompanying notes in our Annual Report on Form 10-K filed with the SEC on March 16, 2006, (ii) the historical financial statements of Javelina and the accompanying notes in our Current Report on Form 8-K/A filed with the SEC on December 22, 2005 and (iii) the historical financial statements of Starfish and the accompanying notes in Amendment No. 1 to our Annual Report on Form 10-K/A filed with the SEC on March 31, 2006.

(2): Financial Statement Schedules: All omitted schedules have been omitted because they are not required or because the required information is contained in the financial statements or notes thereto.
(3): Exhibits:

2.1		Purchase Agreement dated as of March 24, 2003, among PNG Corporation, Energy Spectrum Partners LP, MarkWest Energy GP, L.L.C., MW Texas Limited, L.L.C. and MarkWest Energy Partners, L.P. (Incorporated by reference to Exhibit 2.1 of the Current Report on Form 8-K filed April 14, 2003.)
2.2		Plan of Merger entered into as of March 28, 2003, by and among MarkWest Blackhawk L.P., MarkWest Pinnacle L.P., MarkWest PNG Utility L.P., MarkWest Texas PNG Utility L.P., Pinnacle Natural Gas Company, Pinnacle Pipeline Company, PNG Transmission Company and Bright Star Gathering, Inc. (Incorporated by reference to Exhibit 2.2 of the Current Report on Form 8-K filed April 14, 2003.)
2.3		Asset Purchase-and-Sale Agreement dated as of November 18, 2003, by and between American Central Western Oklahoma Gas Company, L.L.C., MarkWest Western Oklahoma Gas Company, L.L.C. and American Central Gas Technologies, Inc. (Incorporated by reference to Exhibit 2.1 of the Current Report on Form 8-K filed December 16, 2003.)
2.4		Purchase and Sale Agreement, dated as of November 7, 2003, by and between Shell Pipeline Company, LP and Equilon Enterprises L.L.C., dba Shell Oil Products US, and MarkWest Michigan Pipeline Company, L.L.C. (Incorporated by reference to Exhibit 2.1 of the Current Report on Form 8-K filed December 31, 2003.)
2.5		Asset Purchase and Sale Agreement and addendum, thereto, dated as of July 1, 2004, by and between American Central Eastern Texas Gas Company Limited Partnership, ACGC Gathering Company, L.L.C. and MarkWest Energy East Texas Gas Company's L.P. (Incorporated by reference to Exhibit 2.1 of the Current Report on form 8-K/A filed September 13, 2004.)
2.5		Purchase and Sale Agreement effective as of January 1, 2005 between MarkWest Energy Partners L.P. and Enterprise Products Operating L.P. (Incorporated by reference to Exhibit 10.1 of the Current Report on Form 8-K filed April 6, 2005.)
2.6		Purchase and Sale Agreement, dated as of September 16, 2005 Among MarkWest Energy Partners and El Paso Corporation (Incorporated by reference to Exhibit 10.1 of the Current Report on Form 8-K filed September 21, 2005.)
2.7		Purchase and Sale Agreement, dated as of September 16, 2005 Among MarkWest Energy Partners and Kerr-McGee Corporation (Incorporated by reference to Exhibit 10.2 of the Current Report on Form 8-K filed September 21, 2005.)

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2.8		Purchase and Sale Agreement, dated as of September 16, 2005 Among MarkWest Energy Partners and Valero Energy Corp. (Incorporated by reference to Exhibit 10.3 of the Current Report on Form 8-K filed September 21, 2005.)
3.1		Certificate of Limited Partnership of MarkWest Energy Partners, L.P. (Incorporated by reference to Exhibit 3.1 of the Registration Statement (No. 33-81780) on Form S-1 filed January 31, 2002).
3.2		Amended and Restated Agreement of Limited Partnership of MarkWest Energy Partners, L.P., dated as of May 24, 2002. (Incorporated by reference to Exhibit 3.1 of the Current Report on Form 8-K filed June 7, 2002.)
3.3		Certificate of Formation of MarkWest Energy Operating Company, L.L.C. (Incorporated by reference to Exhibit 3.3 of the Registration Statement (No. 33-81780) on Form S-1 filed January 31, 2002).
3.4		Amended and Restated Limited Liability Company Agreement of MarkWest Energy Operating Company, L.L.C., dated as of May 24, 2002. (Incorporated by reference to Exhibit 3.2 of the Current Report on Form 8-K filed June 7, 2002.)
3.5		Certificate of Formation of MarkWest Energy GP, L.L.C. (Incorporated by reference to Exhibit 3.5 of the Registration Statement (No. 33-81780) on Form S-1 filed January 31, 2002).
3.6		Amended and Restated Limited Liability Company Agreement of MarkWest Energy GP, L.L.C., dated as of May 24, 2002. (Incorporated by reference to Exhibit 3.3 of the Current Report on Form 8-K filed June 7, 2002.)
3.6		Amendment No. 1 to Amended and Restated Limited Partnership Agreement MarkWest Energy Partners, L.P. (Incorporated by reference to Exhibit 99.1 of the Current Report on Form 8-K filed January 6, 2005.)
3.7		Amendment No. 2 to Amended and Restated Limited Partnership Agreement MarkWest Energy Partners, L.P. (Incorporated by reference to Exhibit 99.1 of the Current Report on Form 8-K filed June 15, 2005.)
4.1		Purchase Agreement dated as of June 13, 2003, by and among MarkWest Energy Partners, L.P. and Tortoise Capital Advisors, LLC as attorney-in-fact for the Purchasers (Incorporated by reference to Exhibit 4.1 of the Current Report on Form 8-K filed June 19, 2003.)
4.2		Registration Rights Agreement dated as of June 13, 2003, by and among MarkWest Energy Partners, L.P. and Tortoise Capital Advisors, LLC as attorney-in-fact for the Purchasers (Incorporated by reference to Exhibit 4.2 of the Current Report on Form 8-K filed June 19, 2003.)
4.3		Unit Purchase Agreement dated as of July 29, 2004 among MarkWest Energy Partners, L.P., and MarkWest Energy GP, L.L.C. and each of Kayne Anderson Energy Fund I, L.P., and MarkWest Energy GP, L.L.C. and each of Kayne Anderson Energy Fund I, L.P., Kayne Anderson Capital Income Partners, L.P., Kayne Anderson MLP Fund, L.P., Kayne Anderson Capital Income Fund, LTD., Kayne Anderson Income Partners, L.P., HFR RV Performance Master Trust, Tortoise Energy Infrastructure Corporation and Energy Income and Growth Fund as Purchasers (Incorporated by reference to Exhibit 4.1 of the Current Report on form 8-K/A filed September 13, 2004.)

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4.4		Registration Rights Agreement dated as of July 29, 2004, among MarkWest Energy Partners, L.P., and each of Kayne Anderson Energy Fund I, L.P., Kayne Anderson Capital Income Fund, LTD., Kayne Anderson Income Partners, L.P., HFR RV Performance Master Trust, Tortoise Energy Infrastructure Corporation and Energy Income and Growth Fund (Incorporated by reference to Exhibit 4.2 of the Current Report on form 8-K/A filed September 13, 2004.)
4.5		Underwriting Agreement dated as of September 15, 2004, by and among the Partnership, the underwriters named therein and the other parties thereto related to the Common Units Offering (Incorporated by reference to Exhibit 1.1 of the Current Report on Form 8-K filed September 20, 2004.)
4.6		Purchase Agreement dated October 19, 2004, among MarkWest Energy Partners, L.P., MarkWest Energy Finance Corporation, the Guarantors named therein and the Initial Purchasers named therein (Incorporated by reference to Exhibit 4.1 of the Current Report on Form 8-K filed October 25, 2004.)
4.7		Registration Rights Agreement dated October 25, 2004, among MarkWest Energy Partners, L.P., MarkWest Energy Finance Corporation, the Guarantors named therein and the Initial Purchasers named therein (Incorporated by reference to Exhibit 4.3 of the Current Report on Form 8-K filed October 25, 2004.)
4.8		Registration Rights Agreement, dated as of November 9, 2005 (Incorporated by reference to Exhibit 99.3 of the Current Report on Form 8-K filed November 16, 2005.)
4.9		Unit Purchase Agreement, dated as of November 9, 2005 (Incorporated by reference to Exhibit 99.2 of the Current Report on Form 8-K filed November 16, 2005.)
4.10		Registration Rights Agreement, dated as of December 23, 2005 (Incorporated by reference to Exhibit 99.3 of the Current Report on Form 8-K filed December 29, 2005.)
4.11		Unit Purchase Agreement, dated as of December 23, 2005 (Incorporated by reference to Exhibit 99.2 of the Current Report on Form 8-K filed December 29, 2005.)
5.1*		Opinion of Vinson & Elkins L.L.P. as to the legality of the securities being registered.
8.1*		Opinion of Vinson & Elkins L.L.P. as to certain tax matters.
10.1		Credit Agreement dated as of May 20, 2002, among MarkWest Energy Operating Company, L.L.C (as the Borrower), MarkWest Energy Partners, L.P. (as a Guarantor), and various lenders. (Incorporated by reference to Exhibit 10.1 of the Current Report on Form 8-K filed June 7, 2002.)
10.2		Amended and Restated Credit Agreement dated as of December 1, 2003, among MarkWest Energy Operating Company, L.L.C., as Borrower, MarkWest Energy Partners, L.P., as Guarantor, Royal Bank of Canada, as Administrative Agent, Bank One, NA, as Syndication Agent, and Fortis Capital Corp., as Documentation Agent, to the $140,000,000 Senior Credit Facility (Incorporated by reference to Exhibit 10.1 of the Current Report on Form 8-K filed December 16, 2003.)
10.4		MarkWest Energy Partners, L.P. Long-Term Incentive Plan (Incorporated by reference to Exhibit 10.3 of the Current Report on Form 8-K filed June 7, 2002.)

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10.5		First Amendment to MarkWest Energy Partners, L.P. Long-Term Incentive Plan (Incorporated by reference to Exhibit 10.4 of the Current Report on Form 8-K filed June 7, 2002.)
10.6		Omnibus Agreement dated of May 24, 2002, among MarkWest Hydrocarbon, Inc.; MarkWest Energy GP, L.L.C.; MarkWest Energy Partners, L.P.; and MarkWest Energy Operating Company, L.L.C. (Incorporated by reference to Exhibit 10.5 of the Current Report on Form 8-K filed June 7, 2002.)
10.7+		Fractionation, Storage and Loading Agreement dated as of May 24, 2002, between MarkWest Energy Appalachia, L.L.C. and MarkWest Hydrocarbon, Inc. (Incorporated by reference to Exhibit 10.6 of the Current Report on Form 8-K filed June 7, 2002.)
10.8+		Gas Processing Agreement dated as of May 24, 2002, between MarkWest Energy Appalachia, L.L.C. and MarkWest Hydrocarbon, Inc. (Incorporated by reference to Exhibit 10.7 of the Current Report on Form 8-K filed June 7, 2002.)
10.9+		Pipeline Liquids Transportation Agreement dated as of May 24, 2002, between MarkWest Energy Appalachia, L.L.C. and MarkWest Hydrocarbon, Inc. (Incorporated by reference to Exhibit 10.8 of the Current Report on Form 8-K filed June 7, 2002.)
10.10		Natural Gas Liquids Purchase Agreement dated as of May 24, 2002, between MarkWest Energy Appalachia, L.L.C. and MarkWest Hydrocarbon, Inc. (Incorporated by reference to Exhibit 10.9 of the Current Report on Form 8-K filed June 7, 2002.)
10.11+		Gas Processing Agreement (Maytown) dated as of May 28, 2002, between Equitable Production Company and MarkWest Hydrocarbon, Inc. (Incorporated by reference to Exhibit 10.9 of the Registration Statement on Form S-1/A filed May 17, 2002.)
10.12		Amendment to Gas Processing Agreement (Maytown) dated as of March 26, 2002, between Equitable Production Company and MarkWest Hydrocarbon, Inc. (Incorporated by reference to Exhibit 10.10 of the Registration Statement on Form S-1/A filed May 17, 2002.)
10.13		Services Agreement dated January 1, 2004 between MarkWest Energy GP, L.L.C. and MarkWest Hydrocarbon, Inc. (Incorporated by reference to Exhibit 10.13 of the Current Report on Form 10-K filed March 15, 2004.)
10.14		Second Amended and Restated Credit Agreement dated as of July 30, 2004 among MarkWest Energy Operating Company, L.L.C., as Borrower, MarkWest Energy Partners, L.P., as Guarantor, Royal Bank of Canada, as Administrative Agent, Fortis Capital Corp., as Syndication Agent, Bank One, NA, as Documentation Agent and Societe Generale, as Documentation Agent to the $315,000,000 Senior Credit Facility (Incorporated by reference to Exhibit 99.1 of the Current Report on form 8-K/A filed September 13, 2004.)
10.15		First Amendment to the Second Amended and Restated Credit Agreement dated as of August 20, 2004, among MarkWest Energy Operating Company, L.L.C., as Borrower, MarkWest Energy Partners, L.P., as Guarantor, Royal Bank of Canada, as Administrative Agent, Fortis Capital Corp., as Syndication Agent, Bank One, NA, as Documentation Agent and Societe Generale, as Documentation Agent (Incorporated by reference to Exhibit 99.2 of the Current Report on Form 8-K/A filed September 13, 2004.)

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10.16		Third Amended and Restated Credit Agreement dated as of October 25, 2004 among MarkWest Energy Operating Company, L.L.C., as Borrower, MarkWest Energy Partners, L.P., as Guarantor, Royal Bank of Canada, as Administrative Agent, Bank One, N.A., as Syndication Agent, Fortis Capital Corp., as Documentation Agent, U.S. Bank National Association, as Documentation Agent, Societe Generale, as Documentation Agent, and Wachovia Bank, National Association, as Documentation Agent, RBC Capital Markets and J.P. Morgan Securities Inc., as Lead Arrangers and Joint Bookrunners, to the $200,000,000 Senior Credit Facility (Incorporated by reference to Exhibit 99.1 of the Current Report on Form 8-K filed October 29, 2004.)
10.17		MarkWest Hydrocarbon, Inc. 1997 Severance Plan (Incorporated by reference to Exhibit 10.1 of the Quarterly Report of MarkWest Hydrocarbon, Inc. on Form 10-Q filed November 13, 1997.)
10.18		Executive Employment Agreement effective November 1, 2003 between MarkWest Hydrocarbon, Inc. and Frank Semple (Incorporation by reference to Exhibit 10.21 the Annual Report of MarkWest Hydrocarbon, Inc. on Form 10-K filed March 30, 2004.)
10.19		Fourth Amended and Restated Credit Agreement, dated as of November 1, 2005 (Incorporated by reference to Exhibit 99.2 of the Current Report on Form 8-K filed November 7, 2005.)
10.20		Fifth amended and restated credit agreement dated as of December 29, 2005, among MarkWest Energy Operating Company, L.L.C., as Borrower, MarkWest Energy Partners, L.P., as Guarantor, Royal Bank of Canada, as Administrative Agent, Bank One, N.A., as Syndication Agent, Forties Capital Corp., as Documentation Agent, U.S. Bank National Association, as Documentation Agent, Society General, as Documentation Agent, and Wachovia Bank, National Association, as Documentation Agent, RBC Capital Markets and J.P. Morgan Securities Inc., as Lead Arrangers and Joint Bookrunners, to the $615,000,000 Senior Credit Facility (Incorporated by reference to Exhibit 99.1 of the Current Report on Form 8-K filed January 5, 2006.)
10.21+		Construction, Operation and Gas Gathering Agreement dated as of September 21, 2006 between MarkWest Western Oklahoma Gas Company LLC and Newfield Exploration Mid-Continent Inc. (Incorporated by reference to Exhibit 10.1 of the Quarterly Report filed November 7, 2006.)
10.22		Summary of the 2006 Long Term Incentive Plan and Target Percentages (Incorporated by reference to the Current Report on Form 8-K filed November 1, 2006).
16.1		Changes in registrants certifying accountants. MarkWest Energy Partners, L.P. dismissed PricewaterhouseCoopers LLP as its independent accountants (Incorporated by reference to Exhibit 16 of the Current Report on Form 8-K filed March 1, 2004.)
16.2		Changes in registrants certifying accountants. MarkWest Energy Partners, L.P. dismissed KPMG LLP as the Partnership's independent registered public accounting firm and engaged Deloitte & Touche LLP as its new independent registered public accounting firm (Incorporated by reference to Exhibit 16 of the Current Report on Form 8-K filed September 23, 2005.)
21.1		List of subsidiaries (Incorporated by reference to Exhibit 21.1 to our Annual Report on Form 10-K filed March 6, 2006.)

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23.1**		Consent of KPMG LLP
23.2**		Consent of PricewaterhouseCoopers LLP
23.3**		Consent of PricewaterhouseCoopers LLP
23.4**		Consent of Deloitte & Touche LLP
23.5*		Consent of Vinson & Elkins L.L.P. (contained in Exhibits 5.1)
24.1*		Power of Attorney (contained on the signature pages of the initial filing of this registration statement).

+: Application has been made to the Securities and Exchange Commission for confidential treatment of certain provisions of these exhibits. Omitted material for which confidential treatment has been requested and has been filed separately with the Securities and Exchange Commission.
*: Previously filed.
**: Filed herewith.
(b): The following exhibits are filed as part of this report: See Item 15(a)(2) above.
(c): The following financial statement schedules are filed as part of this report: None required.

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