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TABLE OF CONTENTS

As filed with the Securities and Exchange Commission on November 15, 2006

Registration No. 333-            

SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549

FORM S-3
REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

MARKWEST ENERGY PARTNERS, L.P.*
MARKWEST ENERGY FINANCE CORP.
(Exact name of registrant as specified in its charter)

Delaware Delaware	27-0005456 20-1805917
(State or other jurisdictions of incorporation or organization)	(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
MarkWest Energy GP, L.L.C.
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, Suite 3600
Houston, Texas 77002
(713) 758-2222

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

        If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box. o

        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, other than securities offered only in connection with dividend or interest reinvestment plans, 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 registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box. ý

        If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box. o

CALCULATION OF REGISTRATION FEE

Title of each class of securities to be registered			Amount to be Registered/Proposed Maximum Offering Price per Unit/Proposed Maximum Aggregate Offering Price/Amount of Registration Fee(1)
	Common Units
	Debt Securities
	Guarantees of Debt Securities(2)
		Total

(1)

An indeterminate aggregate offering price or number of the securities of each identified class is being registered as may be issued from time to time at indeterminate prices. Separate consideration may or may not be received for securities that are being registered that are issued in exchange for, or upon conversion or exercise of, the debt securities being registered hereunder. In accordance with Rules 456(b) and 457(r), the registrant is deferring payment of all of the registration fee.

(2)

If a series of debt securities is guaranteed, such series will be guaranteed by all subsidiaries other than "minor" subsidiaries as such term is interpreted in securities regulations governing financial reporting for guarantors. Pursuant to Rule 457(n), no separate fee is payable with respect to the guarantees of the debt securities being registered.

        The following are co-registrants that may guarantee the debt securities:

MarkWest Energy Operating Company, L.L.C. (Exact Name of Registrant As Specified In Its Charter)
Delaware (State or Other Jurisdiction of Incorporation or Organization)	27-0005448 (I.R.S. Employer Identification Number)
Basin Pipeline L.L.C. (Exact Name of Registrant As Specified In Its Charter)
Michigan (State or Other Jurisdiction of Incorporation or Organization)	38-3297919 (I.R.S. Employer Identification Number)
West Shore Processing Company, L.L.C. (Exact Name of Registrant As Specified In Its Charter)
Michigan (State or Other Jurisdiction of Incorporation or Organization)	76-0501616 (I.R.S. Employer Identification Number)
MarkWest Energy Appalachia, L.L.C. (Exact Name of Registrant As Specified In Its Charter)
Delaware (State or Other Jurisdiction of Incorporation or Organization)	47-0852385 (I.R.S. Employer Identification Number)
MarkWest Texas GP, L.L.C. (Exact Name of Registrant As Specified In Its Charter)
Delaware (State or Other Jurisdiction of Incorporation or Organization)	30-0157456 (I.R.S. Employer Identification Number)
MW Texas Limited, L.L.C. (Exact Name of Registrant As Specified In Its Charter)
Delaware (State or Other Jurisdiction of Incorporation or Organization)	30-0157458 (I.R.S. Employer Identification Number)
MarkWest Michigan Pipeline Company, L.L.C. (Exact Name of Registrant As Specified In Its Charter)
Michigan (State or Other Jurisdiction of Incorporation or Organization)	83-0375568 (I.R.S. Employer Identification Number)
MarkWest Western Oklahoma Gas Company, L.L.C. (Exact Name of Registrant As Specified In Its Charter)
Oklahoma (State or Other Jurisdiction of Incorporation or Organization)	86-1089780 (I.R.S. Employer Identification Number)

MarkWest New Mexico, L.P. (Exact Name of Registrant As Specified In Its Charter)
Texas (State or Other Jurisdiction of Incorporation or Organization)	30-0157474 (I.R.S. Employer Identification Number)
MarkWest Pinnacle L.P. (Exact Name of Registrant As Specified In Its Charter)
Texas (State or Other Jurisdiction of Incorporation or Organization)	30-0157461 (I.R.S. Employer Identification Number)
MarkWest PNG Utility, L.P. (Exact Name of Registrant As Specified In Its Charter)
Texas (State or Other Jurisdiction of Incorporation or Organization)	30-0157467 (I.R.S. Employer Identification Number)
MarkWest Texas PNG Utility, L.P. (Exact Name of Registrant As Specified In Its Charter)
Texas (State or Other Jurisdiction of Incorporation or Organization)	30-0157468 (I.R.S. Employer Identification Number)
MarkWest Blackhawk, L.P. (Exact Name of Registrant As Specified In Its Charter)
Texas (State or Other Jurisdiction of Incorporation or Organization)	30-157471 (I.R.S. Employer Identification Number)
MarkWest Power Tex L.P. (Exact Name of Registrant As Specified In Its Charter)
Texas (State or Other Jurisdiction of Incorporation or Organization)	45-0537840 (I.R.S. Employer Identification Number)
MarkWest Pipeline Company, L.P. (Exact Name of Registrant As Specified In Its Charter)
Texas (State or Other Jurisdiction of Incorporation or Organization)	20-2229672 (I.R.S. Employer Identification Number)
MarkWest Energy East Texas Gas Company, L.P. (Exact Name of Registrant As Specified In Its Charter)
Delaware (State or Other Jurisdiction of Incorporation or Organization)	20-1311706 (I.R.S. Employer Identification Number)

MarkWest Javelina Company (Exact Name of Registrant As Specified In Its Charter)
Texas (State or Other Jurisdiction of Incorporation or Organization)	76-0263770 (I.R.S. Employer Identification Number)
MarkWest Javelina Holding Company, L.P. (Exact Name of Registrant As Specified In Its Charter)
Delaware (State or Other Jurisdiction of Incorporation or Organization)	Requested (I.R.S. Employer Identification Number)
MarkWest Javelina Pipeline Company (Exact Name of Registrant As Specified In Its Charter)
Texas (State or Other Jurisdiction of Incorporation or Organization)	38-2909338 (I.R.S. Employer Identification Number)
MarkWest Javelina Pipeline Holding, L.P. (Exact Name of Registrant As Specified In Its Charter)
Oklahoma (State or Other Jurisdiction of Incorporation or Organization)	36-4582448 (I.R.S. Employer Identification Number)

PROSPECTUS

MarkWest Energy Partners, L.P.

MarkWest Energy Finance Corp.

Common Units
Debt Securities

        We may from time to time offer the following securities under this prospectus:

•

common units representing limited partner interests in MarkWest Energy Partners, L.P; and



•

debt securities of MarkWest Energy Partners L.P.

        Our common units are traded on the American Stock Exchange under the symbol "MWE." We will provide information in the prospectus supplement for the trading market, for any debt securities we may offer.

        MarkWest Energy Finance Corp. may act as co-issuer of the debt securities, and all other direct or indirect subsidiaries of MarkWest Energy Partners, L.P. (which we refer to as "Subsidiary Guarantors"), other than "minor" subsidiaries as such item is interpreted in securities regulation governing financial reporting for guarantors, may guarantee the debt securities.

        Each time we sell securities we will provide a prospectus supplement that will contain specific information about the terms of that offering. This prospectus may not be used to consummate sales of securities unless accompanied by a prospectus supplement. The prospectus supplement may also add, update or change information contained in this prospectus. You should read this prospectus and any prospectus supplement carefully before you invest. You should also read the documents we have referred you to in the "Where You Can Find More Information" section of this prospectus for information on us and for our financial statements.

Limited partnerships are inherently different from corporations. You should carefully consider each of the factors described under "Risk Factors," which begin on page 1 of this prospectus before you make an investment in our securities.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the securities or passed upon the accuracy or adequacy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is November 15, 2006

TABLE OF CONTENTS

        You should rely only on the information contained or incorporated by reference in this prospectus or any prospectus supplement. We have not authorized any other person to provide you with different information. You should not assume that the information incorporated by reference or provided in this prospectus or any prospectus supplement is accurate as of any date other than the date on the front of each document.

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ABOUT THIS PROSPECTUS

        This prospectus is part of a registration statement that we and MarkWest Energy Finance Corporation have filed with the Securities and Exchange Commission using a "shelf" registration process. Under this shelf registration process, we may offer from time to time our common units or debt securities described in this prospectus in one or more offerings. Each time we offer securities, we will provide you with a prospectus supplement that will describe, among other things, the specific amounts and prices of the securities being offered and the terms of the offering. The prospectus supplement may also add, update or change information contained in this prospectus. Therefore, before you invest in our securities, you should read this prospectus, any attached prospectus supplements and the additional information described below under "Incorporation of Certain Information by Reference".

        As used in this prospectus, "we," "us," "our," "the Partnership" and "MarkWest Energy Partners" mean MarkWest Energy Partners, L.P. and, where the context requires, include our operating subsidiaries.

WHERE YOU CAN FIND MORE INFORMATION

        We file annual, quarterly and special reports and other information with the Securities and Exchange Commission under the Securities Exchange Act of 1934. You can inspect and/or copy these reports and other information at offices maintained by the SEC, including:

•

the principal offices of the SEC located at Judiciary Plaza, 100 F Street, N.E., Washington, D.C. 20549; and



•

The SEC's website at http://www.sec.gov.

        In addition, please call the SEC at 1-800-732-0330 for further information on their public reference room.

        Further, our common units are listed on the American Stock Exchange, and you can inspect similar information at the offices of the American Stock Exchange, located at 86 Trinity Place, New York, New York 10006.

        The SEC allows us to incorporate by reference information we file with it into this prospectus. This procedure means that we can disclose important information to you by referring you to documents on file or to be filed with the SEC. The information we incorporate by reference is part of this prospectus and later information that we file with the SEC will automatically update and supersede this information. Therefore, before you decide to invest in a particular offering under this shelf registration, you should always check for SEC reports we may have filed after the date 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(c), 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 shelf registration are completed:

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Amendments No. 1 and No. 2 to our Annual Report on Form 10-K/A for the year ended December 31, 2005;



•

Our Annual Report on Form 10-K for the year ended December 31, 2005;



•

Amendments No. 1 and No. 2 to our Quarterly Report on Form 10-Q/A for the period ended March 31, 2006;



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Our Quarterly Reports on Form 10-Q for the periods ended March 31, 2006, June 30, 2006 and September 30, 2006;

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•

Our Current Reports on Forms 8-K filed 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;



•

Amendment No. 1 to our Current Report on Form 8-K/A filed December 22, 2005; and



•

The description of our common units contained in our Form 8-A, initially filed with the SEC on February 14, 2002, and any subequent amendment thereto filed for the purpose of updating such description.

        You may request a copy of these filings at no cost by making written or telephone requests for copies to:

MarkWest Energy Partners, L.P.
1515 Arapahoe Street
Tower 2, Suite 700
Denver, Colorado 80202
Attention: Andrew L. Schroeder
Telephone: (303) 925-9200

FORWARD-LOOKING STATEMENTS

        Statements included in this prospectus which are not historical facts (including any statements concerning plans and objectives of management for future operations or economic performance, or assumptions related thereto) are forward-looking statements. In addition, we and our representatives may from time to time make other oral or written statements which are also forward-looking statements.

        These forward-looking statements are made based upon management's current plans, 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:

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Our ability to successfully integrate our recent or future acquisitions;



•

The availability of raw natural gas supply for our gathering and processing services;



•

Our substantial debt and other financial obligations could adversely impact our financial condition;



•

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 natural gas exploration and production activities will not occur or be successful;



•

Prices of NGL products and crude oil, including the effectiveness of any hedging activities, and indirectly natural gas prices;



•

Competition from other NGL processors and natural gas gatherers, including major energy companies;

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Changes in general economic conditions in regions in which our products are located;



•

Our ability to identify and consummate grass roots projects or acquisitions complementary to our business; and.



•

Our ability to raise sufficient capital to execute our business plan through borrowing or issuing equity.

        These factors are not necessarily all of the important factors that could cause actual results to differ materially from those expressed in any of our forward-looking statements. Other unknown or unpredictable factors could also have material adverse effects on future results. The Partnership undertakes no obligation to update publicly any forward-looking statement whether as a result of new information or future events.

        For a more detailed description of these particular forward-looking statements and for other factors that may affect any forward-looking statements, see the section entitled "Management's Discussion and Analysis of Financial Condition and Results of Operations" in Item 7 of our most recently filed Annual Report on Form 10-K, as incorporated herein by reference.

        When considering any forward-looking statement, you should also keep in mind the risk factors described under the section entitled "Risk Factors" beginning on page 1 of this prospectus and any other risk factors described in an applicable prospectus supplement. Investors are cautioned not to put undue reliance on forward-looking statements.

ABOUT MARKWEST ENERGY PARTNERS

        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 2, Suite 700, Denver, Colorado 80202, 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.

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RISK FACTORS

You should carefully consider the following risk factors together with all of the other information included in this prospectus, any prospectus supplement and the information that we have incorporated herein by reference in evaluating an investment in MarkWest Energy Partners, L.P. If any of the following risks were actually to occur, our business, financial condition or results of operations could be materially adversely affected. When we offer and sell any securities pursuant to a prospectus supplement, we may include additional risk factors relevant to such securities in the prospectus supplement.

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;



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the volumes of natural gas we gather, process and transport;



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the level of our operating costs, including reimbursement of fees and expenses of our general partner; and



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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:

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our debt service requirements;



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fluctuations in our working capital needs;



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our ability to borrow funds and access capital markets;



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restrictions contained in our debt agreements;



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the level of capital expenditures we make, including capital expenditures incurred in connection with our enhancement projects;



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the cost of acquisitions, if any; and



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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

1

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 future acquisitions, our future financial performance may suffer.

        Our future growth will depend in part on our ability to integrate our future acquisitions. We cannot guarantee that we will successfully integrate any 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:

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operating a significantly larger combined organization and integrating additional midstream operations into our existing operations;



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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;



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the loss of customers or key employees from the acquired businesses;



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the diversion of management's attention from other existing business concerns;



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the failure to realize expected synergies and cost savings;



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coordinating geographically disparate organizations, systems and facilities;



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integrating personnel from diverse business backgrounds and organizational cultures; and



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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 to 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

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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:

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make it more difficult for us to satisfy our obligations with respect to our existing debt;



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impair our ability to obtain additional financings in the future for working capital, capital expenditures, acquisitions, or general partnership and other purposes;



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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;



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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;



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limit our flexibility in planning for, or reacting to, changes in our business and the industry in which we operate; and



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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 out 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 the credit facility, make distributions on equity investments, and declare or make, directly or indirectly, any distribution on our common units. Please read "Item 7. Management's Discussion and Analysis of Financial Condition and Results of Operations — Liquidity and Capital Resources" as set forth in our most recent Annual Report on Form 10-K incorporated by reference herein. 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. In particular, we may be unable to meet those ratios and conditions. Any future breach of any of these

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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. Natural gas prices reached historic highs in 2005 and early 2006 but have declined in the second half of 2006. These recent declines in natural gas prices are beginning to have a negative impact on production activity, and if sustained, could lead to a material decrease in such production activity and ultimately to exploration activity.

        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.

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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.

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        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 "Item 1. 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 potential existence of a 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.

        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

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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. Please read "Item 7. Management's Discussion and Analysis of Financial Condition and Results of Operations — Quantitative and Qualitative Disclosures about Market Risk" as set forth in our most recent Annual Report on Form 10-K incorporated by reference herein. 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 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.

        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

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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

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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

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other regulatory matters that could affect our business, please read "Item 1. Business — Regulatory Matters," "Item 1. Business — Environmental Matters," and "Item 1. 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.

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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 loss to both offshore and onshore assets resulting from the hurricanes has 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 our coverage and broadening the self-insurance element of our overall coverage. In the future, we may be unable to obtain adequate 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.

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•

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.

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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.

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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.

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        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.

15

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

16

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. If you are a tax exempt entity or a foreign person, you should consult your tax advisor before investing in our common units.

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.

17

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.

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USE OF PROCEEDS

        Unless we specify otherwise in any prospectus supplement, we will use the net proceeds we receive from the sale of securities covered by this prospectus for general corporate purposes, which may include, among other things:

•

paying or refinancing all or a portion of our indebtedness outstanding at the time; and



•

funding working capital, capital expenditures or acquisitions.

        The actual application of proceeds from the sale of any particular offering of securities using this prospectus will be described in the applicable prospectus supplement relating to such offering. The precise amount and timing of the application of these proceeds will depend upon our funding requirements and the availability and cost of other funds.

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RATIOS OF EARNINGS TO FIXED CHARGES

        The table below sets forth the Ratios of Earnings to Fixed Charges for the Partnership for the periods indicated. On May 24, 2002, the Partnership completed its initial public offering whereby the Partnership became successor to the business of the MarkWest Hydrocarbon Midstream Business (Midstream Business). As such, the period ended December 31, 2001 reflects the financial results of the Midstream Business.

	Year Ended December 31,
											Nine Months Ended September 30, 2006
	2001		2002		2003		2004		2005
RATIO OF EARNINGS TO FIXED CHARGES	3.79	x	3.77	x	2.07	x	1.57	x	1.15	x	2.35	x

        For purposes of computing the ratio of earnings to fixed charges, "earnings" is the aggregate of the following items:

•

pre-tax income or loss from continuing operations before adjustment for minority interests in consolidated subsidiaries or income or loss from equity investees;



•

plus fixed charges;



•

plus distributed income of equity investees;



•

less capitalized interest; and



•

less minority interest in pre-tax income of subsidiaries that have not incurred fixed charges.

        The term "fixed charges" means the sum of the following:

•

interest expense and capitalized, including amortization of premiums, discounts and capitalized expenses related to indebtedness (excluding write-off of capitalized expenses related to indebtedness); and



•

an estimate of the interest within rental expense.

20

DESCRIPTION OF OUR DEBT SECURITIES

General

        The debt securities will be:

•

our direct general obligations;



•

either senior debt securities or subordinated debt securities; and



•

issued under separate indentures among us and Wells Fargo Bank, National Association, as Trustee.

        MarkWest Energy Partners, L.P. may issue debt securities in one or more series, and MarkWest Energy Finance Corp. may be a co-issuer of one or more series of debt securities. MarkWest Energy Finance Corp. was incorporated under the laws of the State of Delaware in 2004, is wholly-owned by MarkWest Energy Partners, L.P., and has no material assets or any liabilities other than as a co-issuer of debt securities. Its activities will be limited to co-issuing debt securities and engaging in other activities incidental thereto. When used in this section "Description of the Debt Securities," the terms "we," "us," "our" and "issuers" refer jointly to MarkWest Energy Partners, L.P. and MarkWest Energy Finance Corp., and the terms "MarkWest Energy Partners" and "MarkWest Finance" refer strictly to MarkWest Energy Partners, L.P. and MarkWest Energy Finance Corp., respectively.

        If we offer senior debt securities, we will issue them under a senior indenture. If we issue subordinated debt securities, we will issue them under a subordinated indenture. A form of each indenture is filed as an exhibit to the registration statement of which this prospectus is a part. We have not restated either indenture in its entirety in this description. You should read the relevant indenture because it, and not this description, controls your rights as holders of the debt securities. Capitalized terms used in the summary have the meanings specified in the indentures.

Specific Terms of Each Series of Debt Securities in the Prospectus Supplement

        A prospectus supplement and a supplemental indenture or authorizing resolutions relating to any series of debt securities being offered will include specific terms relating to the offering. These terms will include some or all of the following:

•

whether MarkWest Finance will be a co-issuer;



•

the guarantors of the debt securities, if any;



•

whether the debt securities are senior or subordinated debt securities;



•

the title of the debt securities;



•

the total principal amount of the debt securities;



•

the assets, if any, that are pledged as security for the payment of the debt securities;



•

whether we will issue the debt securities in individual certificates to each holder in registered form, or in the form of temporary or permanent global securities held by a depository on behalf of holders;



•

the prices at which we will issue the debt securities;



•

the portion of the principal amount that will be payable if the maturity of the debt securities is accelerated;



•

the currency or currency unit in which the debt securities will be payable, if not U.S. dollars;



•

the dates on which the principal of the debt securities will be payable;

21

•

the interest rate that the debt securities will bear and the interest payment dates for the debt securities;



•

any conversion or exchange provisions;



•

any optional redemption provisions;



•

any sinking fund or other provisions that would obligate us to repurchase or otherwise redeem the debt securities;



•

any changes to or additional events of default or covenants; and



•

any other terms of the debt securities.

        We may offer and sell debt securities, including original issue discount debt securities, at a substantial discount below their principal amount. The prospectus supplement will describe special U.S. federal income tax and any other considerations applicable to those securities. In addition, the prospectus supplement may describe certain special U.S. federal income tax or other considerations applicable to any debt securities that are denominated in a currency other than U.S. dollars.

Guarantees

        If specified in the prospectus supplement respecting a series of debt securities, the subsidiaries of MarkWest Energy Partners specified in the prospectus supplement will unconditionally guarantee to each holder and the Trustee, on a joint and several basis, the full and prompt payment of principal of, premium, if any, and interest on the debt securities of that series when and as the same become due and payable, whether at stated maturity, upon redemption or repurchase, by declaration of acceleration or otherwise. If a series of debt securities is guaranteed, such series will be guaranteed by all subsidiaries other than "minor" subsidiaries as such term is interpreted in securities regulation governing financial reporting for guarantors. The prospectus supplement will describe any limitation on the maximum amount of any particular guarantee and the conditions under which guarantees may be released.

        The guarantees will be general obligations of the guarantors. Guarantees of subordinated debt securities will be subordinated to the Senior Indebtedness of the guarantors on the same basis as the subordinated debt securities are subordinated to the Senior Indebtedness of MarkWest Energy Partners.

Consolidation, Merger or Asset Sale

        Each indenture will, in general, allow us to consolidate or merge with or into another domestic entity. It will also allow each issuer to sell, lease, transfer or otherwise dispose of all or substantially all of its assets to another domestic entity. If this happens, the remaining or acquiring entity must assume all of the issuer's responsibilities and liabilities under the indenture including the payment of all amounts due on the debt securities and performance of the issuer's covenants in the indenture.

        However, each indenture will impose certain requirements with respect to any consolidation or merger with or into an entity, or any sale, lease, transfer or other disposition of all or substantially all of an issuer's assets, including:

•

the remaining or acquiring entity must be organized under the laws of the United States, any state or the District of Columbia;



•

the remaining or acquiring entity must assume the issuer's obligations under the indenture; and



•

immediately after giving effect to the transaction, no Default or Event of Default (as defined under "—Events of Default and Remedies" below) may exist.

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        The remaining or acquiring entity will be substituted for the issuer in the indenture with the same effect as if it had been an original party to the indenture, and the issuer will be relieved from any further obligations under the indenture.

No Protection in the Event of a Change of Control

        Unless otherwise set forth in the prospectus supplement, the debt securities will not contain any provisions that protect the holders of the debt securities in the event of a change of control of us or in the event of a highly leveraged transaction, whether or not such transaction results in a change of control of us.

Modification of Indentures

        We may supplement or amend an indenture if the holders of a majority in aggregate principal amount of the outstanding debt securities of all series issued under the indenture affected by the supplement or amendment consent to it. Further, the holders of a majority in aggregate principal amount of the outstanding debt securities of any series may waive past defaults under the indenture and compliance by us with our covenants with respect to the debt securities of that series only. Those holders may not, however, waive any default in any payment on any debt security of that series or compliance with a provision that cannot be supplemented or amended without the consent of each holder affected. Without the consent of each outstanding debt security affected, no modification of the indenture or waiver may:

•

reduce the principal amount of debt securities whose holders must consent to an amendment, supplement or waiver;



•

reduce the principal of or change the fixed maturity of any debt security;



•

reduce or waive the premium payable upon redemption or alter or waive the provisions with respect to the redemption of the debt securities (except as may be permitted in the case of a particular series of debt securities);



•

reduce the rate of or change the time for payment of interest on any debt security;



•

waive a Default or an Event of Default in the payment of principal of or premium, if any, or interest on the debt securities (except a rescission of acceleration of the debt securities by the holders of at least a majority in aggregate principal amount of the debt securities and a waiver of the payment default that resulted from such acceleration);



•

except as otherwise permitted under the indenture, release any security that may have been granted with respect to the debt securities;



•

make any debt security payable in currency other than that stated in the debt securities;



•

in the case of any subordinated debt security, make any change in the subordination provisions that adversely affects the rights of any holder under those provisions;



•

make any change in the provisions of the indenture relating to waivers of past Defaults or the rights of holders of debt securities to receive payments of principal of or premium, if any, or interest on the debt securities;



•

waive a redemption payment with respect to any debt security (except as may be permitted in the case of a particular series of debt securities);



•

except as otherwise permitted in the indenture, release any guarantor from its obligations under its guarantee or the indenture or change any guarantee in any manner that would adversely affect the rights of holders; or

23

•

make any change in the preceding amendment, supplement and waiver provisions (except to increase any percentage set forth therein).

        We may supplement or amend an indenture without the consent of any holders of the debt securities in certain circumstances, including:

•

to establish the form of terms of any series of debt securities;



•

to cure any ambiguity, defect or inconsistency;



•

to provide for the assumption of an issuer's or guarantor's obligations to holders of debt securities in the case of a merger or consolidation or disposition of all or substantially all of such issuer's or guarantor's assets;



•

in the case of any subordinated debt security, to make any change in the subordination provisions that limits or terminates the benefits applicable to any holder of Senior Indebtedness of MarkWest Energy Partners;



•

to add or release guarantors pursuant to the terms of the indenture;



•

to make any changes that would provide any additional rights or benefits to the holders of debt securities or that do not, taken as a whole, adversely affect the rights under the indenture of any holder of debt securities;



•

to evidence or provide for the acceptance of appointment under the indenture of a successor Trustee;



•

to add any additional Events of Default; or



•

to secure the debt securities and/or the guarantees.

Events of Default and Remedies

        "Event of Default," when used in an indenture, will mean any of the following with respect to the debt securities of any series:

•

failure to pay when due the principal of or any premium on any debt security of that series;



•

failure to pay, within 60 days of the due date, interest on any debt security of that series;



•

failure to pay when due any sinking fund payment with respect to any debt securities of that series;



•

failure on the part of the issuers to comply with the covenant described under "—Consolidation, Merger or Asset Sale";



•

failure to perform any other covenant in the indenture that continues for 30 days after written notice is given to the issuers;



•

certain events of bankruptcy, insolvency or reorganization of an issuer; or



•

any other Event of Default provided under the terms of the debt securities of that series.

        An Event of Default for a particular series of debt securities will not necessarily constitute an Event of Default for any other series of debt securities issued under an indenture. The Trustee may withhold notice to the holders of debt securities of any default (except in the payment of principal, premium, if any, or interest) if it considers such withholding of notice to be in the best interests of the holders.

        If an Event of Default for any series of debt securities occurs and continues, the Trustee or the holders of at least 25% in aggregate principal amount of the debt securities of the series may declare

24

the entire principal of, and accrued interest on, all the debt securities of that series to be due and payable immediately. If this happens, subject to certain conditions, the holders of a majority in the aggregate principal amount of the debt securities of that series can rescind the declaration.

        Other than its duties in case of a default, a Trustee is not obligated to exercise any of its rights or powers under either indenture at the request, order or direction of any holders, unless the holders offer the Trustee reasonable security or indemnity. If they provide this reasonable security or indemnification, the holders of a majority in aggregate principal amount of any series of debt securities may direct the time, method and place of conducting any proceeding or any remedy available to the Trustee, or exercising any power conferred upon the Trustee, for that series of debt securities.

No Limit on Amount of Debt Securities

        Neither indenture will limit the amount of debt securities that we may issue, unless we indicate otherwise in a prospectus supplement. Each indenture will allow us to issue debt securities of any series up to the aggregate principal amount that we authorize.

Registration of Notes

        We will issue debt securities of a series only in registered form, without coupons, unless otherwise indicated in the prospectus supplement.

Minimum Denominations

        Unless the prospectus supplement states otherwise, the debt securities will be issued only in principal amounts of $1,000 each or integral multiples of $1,000.

No Personal Liability

        None of the past, present or future partners, incorporators, managers, members, directors, officers, employees, unitholders or stockholders of either issuer, the general partner of MarkWest Energy Partners or any guarantor will have any liability for the obligations of the issuers or any guarantors under either indenture or the debt securities or for any claim based on such obligations or their creation. Each holder of debt securities by accepting a debt security waives and releases all such liability. The waiver and release are part of the consideration for the issuance of the debt securities. The waiver may not be effective under federal securities laws, however.

Payment and Transfer

        The Trustee will initially act as paying agent and registrar under each indenture. The issuers may change the paying agent or registrar without prior notice to the holders of debt securities, and the issuers or any of their subsidiaries may act as paying agent or registrar.

        If a holder of debt securities has given wire transfer instructions to the issuers, the issuers will make all payments on the debt securities in accordance with those instructions. All other payments on the debt securities will be made at the corporate trust office of the Trustee, unless the issuers elect to make interest payments by check mailed to the holders at their addresses set forth in the debt security register.

        The Trustee and any paying agent will repay to us upon request any funds held by them for payments on the debt securities that remain unclaimed for two years after the date upon which that payment has become due. After payment to us, holders entitled to the money must look to us for payment as general creditors.

25

Exchange, Registration and Transfer

        Debt securities of any series will be exchangeable for other debt securities of the same series, the same total principal amount and the same terms but in different authorized denominations in accordance with the indenture. Holders may present debt securities for exchange or registration of transfer at the office of the registrar. The registrar will effect the transfer or exchange when it is satisfied with the documents of title and identity of the person making the request. We will not charge a service charge for any registration of transfer or exchange of the debt securities. We may, however, require the payment of any tax or other governmental charge payable for that registration.

        We will not be required:

•

to issue, register the transfer of, or exchange debt securities of a series either during a period beginning 15 business days prior to the selection of debt securities of that series for redemption and ending on the close of business on the day of mailing of the relevant notice of redemption or repurchase, or between a record date and the next succeeding interest payment date; or



•

to register the transfer of or exchange any debt security called for redemption or repurchase, except the unredeemed portion of any debt security we are redeeming or repurchasing in part.

Provisions Relating only to the Senior Debt Securities

        The senior debt securities will rank equally in right of payment with all of our other senior and unsubordinated debt. The senior debt securities will be effectively subordinated, however, to all of our secured debt to the extent of the value of the collateral for that debt. We will disclose the amount of our secured debt in the prospectus supplement.

Provisions Relating only to the Subordinated Debt Securities

Subordinated Debt Securities Subordinated to Senior Indebtedness

        The subordinated debt securities will rank junior in right of payment to all of the Senior Indebtedness of MarkWest Energy Partners. "Senior Indebtedness" will be defined in a supplemental indenture or authorizing resolutions respecting any issuance of a series of subordinated debt securities, and the definition will be set forth in the prospectus supplement.

Payment Blockages

        The subordinated indenture will provide that no payment of principal, interest and any premium on the subordinated debt securities may be made in the event:

•

we or our property is involved in any voluntary or involuntary liquidation or bankruptcy;



•

we fail to pay the principal, interest, any premium or any other amounts on any Senior Indebtedness of MarkWest Energy Partners within any applicable grace period or the maturity of such Senior Indebtedness is accelerated following any other default, subject to certain limited exceptions set forth in the subordinated indenture; or



•

any other default on any Senior Indebtedness of MarkWest Energy Partners occurs that permits immediate acceleration of its maturity, in which case a payment blockage on the subordinated debt securities will be imposed for a maximum of 179 days at any one time.

No Limitation on Amount of Senior Debt

        The subordinated indenture will not limit the amount of Senior Indebtedness that MarkWest Energy Partners may incur, unless otherwise indicated in the prospectus supplement.

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Book Entry, Delivery and Form

        The debt securities of a particular series may be issued in whole or in part in the form of one or more global certificates that will be deposited with the Trustee as custodian for The Depository Trust Company, New York, New York ("DTC"), and registered in the name of DTC's nominee, Cede & Co. This means that we will not issue certificates to each holder. Instead, one or more global debt securities will be issued to DTC, who will keep a computerized record of its participants (for example, your broker) whose clients have purchased the debt securities. The participant will then keep a record of its clients who purchased the debt securities. Unless it is exchanged in whole or in part for a certificated debt security, a global debt security may not be transferred, except that DTC, its nominees and their successors may transfer a global debt security as a whole to one another.

        Beneficial interests in global debt securities will be shown on, and transfers of beneficial interests in global debt securities will be made only through, records maintained by DTC and its participants.

        DTC has provided us the following information: DTC is a limited-purpose trust company organized under the New York Banking Law, a "banking organization" within the meaning of the New York Banking Law, a member of the United States Federal Reserve System, a "clearing corporation" within the meaning of the New York Uniform Commercial Code and a "clearing agency" registered under the provisions of Section 17A of the Securities Exchange Act of 1934. DTC holds securities that its participants ("Direct Participants") deposit with DTC. DTC also records the settlement among Direct Participants of securities transactions, such as transfers and pledges, in deposited securities through computerized records for Direct Participants' accounts. This eliminates the need to exchange certificates. Direct Participants include securities brokers and dealers, banks, trust companies, clearing corporations and certain other organizations.

        DTC's book-entry system is also used by other organizations such as securities brokers and dealers, banks and trust companies that work through a Direct Participant. The rules that apply to DTC and its Direct Participants are on file with the SEC.

        DTC is owned by a number of its Direct Participants and by the New York Stock Exchange, Inc., The American Stock Exchange, Inc. and the National Association of Securities Dealers, Inc.

        We will wire all payments on the global debt securities to DTC's nominee. We and the Trustee will treat DTC's nominee as the owner of the global debt securities for all purposes. Accordingly, we, the Trustee and any paying agent will have no direct responsibility or liability to pay amounts due on the global debt securities to owners of beneficial interests in the global debt securities.

        It is DTC's current practice, upon receipt of any payment on the global debt securities, to credit Direct Participants' accounts on the payment date according to their respective holdings of beneficial interests in the global debt securities as shown on DTC's records. In addition, it is DTC's current practice to assign any consenting or voting rights to Direct Participants whose accounts are credited with debt securities on a record date, by using an omnibus proxy. Payments by Direct Participants to owners of beneficial interests in the global debt securities, and voting by Direct Participants, will be governed by the customary practices between the Direct Participants and owners of beneficial interests, as is the case with debt securities held for the account of customers registered in "street name." However, payments will be the responsibility of the Direct Participants and not of DTC, the Trustee or us.

        Debt securities represented by a global debt security will be exchangeable for certificated debt securities with the same terms in authorized denominations only if:

•

DTC notifies us that it is unwilling or unable to continue as depositary or if DTC ceases to be a clearing agency registered under applicable law and in either event a successor depositary is not appointed by us within 90 days; or

27

•

we determine not to require all of the debt securities of a series to be represented by a global debt security.

Satisfaction and Discharge; Defeasance

        Each indenture will be discharged and will cease to be of further effect as to all outstanding debt securities of any series issued thereunder, when:

        (a)   either:

        (1)   all outstanding debt securities of that series that have been authenticated (except lost, stolen or destroyed debt securities that have been replaced or paid and debt securities for whose payment money has theretofore been deposited in trust and thereafter repaid to us) have been delivered to the Trustee for cancellation; or

        (2)   all outstanding debt securities of that series that have not been delivered to the Trustee for cancellation have become due and payable by reason of the giving of a notice of redemption or otherwise or will become due and payable at their stated maturity within one year or are to be called for redemption within one year under arrangements satisfactory to the Trustee and in any case we have irrevocably deposited or caused to be irrevocably deposited with the Trustee as trust funds in trust cash in U.S. dollars, non-callable U.S. Government Obligations or a combination thereof, in such amounts as will be sufficient, without consideration of any reinvestment of interest, to pay and discharge the entire indebtedness of such debt securities not delivered to the Trustee for cancellation, for principal, premium, if any, and accrued interest to the date of such deposit (in the case of debt securities that have been due and payable) or the stated maturity or redemption date;

        (b)   we have paid or caused to be paid all other sums payable by us under the indenture; and

        (c)   we have delivered an officers' certificate and an opinion of counsel to the Trustee stating that all conditions precedent to satisfaction and discharge have been satisfied.

        The debt securities of a particular series will be subject to legal or covenant defeasance to the extent, and upon the terms and conditions, set forth in the prospectus supplement.

The Trustee

        Wells Fargo Bank, National Association will be the initial Trustee under each indenture. We maintain a banking relation in the ordinary course of business with Wells Fargo, National Association and some of its affiliates.

Limitations on Trustee if it is a Creditor

        Each indenture will limit the right of the Trustee thereunder, in the event that it becomes a creditor of an issuer or guarantor, to obtain payment of claims in certain cases, or to realize on certain property received in respect of any such claim as security or otherwise.

Certificates and Opinions to be Furnished to Trustee

        Each indenture will provide that, in addition to other certificates or opinions that may be specifically required by other provisions of the indenture, every application by us for action by the Trustee must be accompanied by a certificate of certain of our officers and an opinion of counsel (who may be our counsel) stating that, in the opinion of the signers, all conditions precedent to such action have been complied with by us.

Governing Law

        Each indenture and all of the debt securities will be governed by the laws of the State of New York.

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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:

29

•

$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;

30

•

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, 2005 with respect to 20% of the subordinated units;



•

June 30, 2006 with respect to 20% of the subordinated units;



•

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 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.

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        This additional early conversion was a one-time occurrence.

        In addition to the early conversion of subordinated units described above, we expect an additional 20% of the subordinated units to convert 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.

        After the early conversion of an additional 600,000 subordinated units into common units on November 15, 2006, only 600,000 subordinated units will remain outstanding. 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;

32

•

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.

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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.

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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;

35

•

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.

36

        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.

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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 herein 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").

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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. on such matters. It is 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 a partnership and our operating company will be disregarded as an entity separate from us 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 company 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

39

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 or indirect controlling 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.

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        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

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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.

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        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.

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        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 loaning 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 as to property other than certain goodwill properties), 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 under Section 168 of the Internal Revenue Code to be depreciated over the remaining cost recovery period for the Section 704(c) built-in gain. If we elect a method other than the remedial method with respect to a goodwill property, Treasury Regulation Section 1.197-2(g)(3) generally requires that the Section 743(b) adjustment attributable to an amortizable Section 197 intangible, which includes goodwill property, should be treated as a newly-acquired asset placed in service in the month when the purchaser acquires the common unit. 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. If we elect a method other than the remedial method, the depreciation and amortization methods and useful lives associated with the Section 743(b) adjustment, therefore, may differ from the methods and useful lives generally used to depreciate the inside basis in such properties. 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 and any other Treasury Regulations. If we elect a method other than the remedial method with respect to a goodwill property, the common basis of such property is not amortizable. 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 direct or indirect controlling 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

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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 methods employed by other publicly traded partnerships 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, and Treasury Regulation Section 1.197-2(g)(3). 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 unitholder's tax basis for his common units is reduced by his share of our deductions (whether or not such deductions were claimed on an individual's income tax return) so that any position we take that understates deductions will overstate the common unitholder's basis in his common units, which may cause the unitholder to understate gain or overstate loss on any sale of such units. Please read "—Disposition of Common Units—Recognition of Gain or Loss." The IRS may challenge our position with respect to depreciating or amortizing the Section 743(b) adjustment we take to preserve the uniformity of the units. If such a challenge were sustained, the gain from the sale of units might be increased without the benefit of additional deductions.

        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 nonamortizable or 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.

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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. Because our general partner may determine not to adopt the remedial method of allocation with respect to any difference between the tax basis and the fair market value of goodwill immediately prior to this or any future offering, we may not be entitled to any amortization deductions with respect to any goodwill conveyed to us on formation or held by us at the time of any future offering. Please read "—Uniformity of Units". 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.

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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, which generally means that the tax basis allocated to the interest sold equals an amount that bears the same relation to the partner's tax basis in his entire interest in the partnership as the value of the interest sold bears to the value of the partner's entire interest in the partnership. 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;

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•

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 the 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 transferor and transferee 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 transferor and transferee unitholders, as well as unitholders whose interests vary during a taxable year, 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 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.

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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) and Treasury Regulation Section 1.197-2(g)(3). 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, and Treasury Regulation Section 1.197-2(g)(3). 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

49

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

50

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;

        (c)   the amount and description of units held, acquired or transferred for the beneficial owner; and

        (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.

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        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 for partnerships, individuals, S corporations, and trusts in excess of $2 million in any single year, or $4 million in any combination of tax years. 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.

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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.

Tax Consequences of Ownership of Debt Securities

        A description of the material federal income tax consequences of the acquisition, ownership and disposition of debt securities will be set forth on the prospectus supplement relating to the offering of debt securities.

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LEGAL MATTERS

        In connection with particular offerings of the securities in the future, and if stated in the applicable prospectus supplement, the validity of those securities may be passed upon for us by Vinson & Elkins L.L.P. and for any underwriters or agents by counsel named in the applicable prospectus supplement.

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.

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PART II
INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14. Other Expenses of Issuance and Distribution

        Set forth below are the expenses (other than underwriting discounts and commissions) expected to be incurred in connection with the issuance and distribution of the securities registered hereby. With the exception of the Securities and Exchange Commission registration fee, the amounts set forth below are estimates.

Securities and Exchange Commission registration fee		$*
Legal fees and expenses		**
Accounting fees and expenses		**
Printing expenses		**
Listing Fees		***
Miscellaneous		**
	TOTAL	$

*

Under SEC Rule 456(b) and 457(c), the SEC registration fee will be paid at the time of any particular offering of securities under the registration statement, and is therefore not currently determinable.

**

Because an indeterminate amount of securities is covered by this registration statement, the expenses in connection with the issuance and distribution of the securities are therefore not currently determinable.

***

The listing fee is based upon the principal amount of securites listed, if any, and is therefore not currently determinable.

Item 15. Indemnification of Directors and Officers

        Section 17-108 of the Delaware Revised 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. The partnership agreement of MarkWest Energy Partners L.P. provides that, 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 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

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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.

        Any underwriting agreement entered into in connection with the sale of the securities offered pursuant to this registration statement will provide for indemnification of officers and directors of the general partner, including liabilities under the Securities Act.

Item 16. Exhibits

(a)

Exhibits:

Exhibit Number			Description
1.1	*	—	Form of Underwriting Agreement
3.1	(1)	—	Certificate of Limited Partnership of MarkWest Energy Partners, L.P.
3.2	(4)	—	Amended and Restated Agreement of Limited Partnership of MarkWest Energy Partners, L.P. dated as of May 24, 2002
3.3	(1)	—	Certificate of Formation of MarkWest Energy Operating Company, L.L.C.
3.4	(2)	—	Amended and Restated Limited Liability Company Agreement of MarkWest Energy Operating Company, L.L.C. dated as of May 24, 2002
3.5	(1)	—	Certificate of Formation of MarkWest Energy GP, L.L.C.
3.6	(3)	—	Amended and Restated Limited Liability Company Agreement of MarkWest Energy GP, L.L.C. dated as of May 24, 2002
4.1	**	—	Form of Senior Indenture of MarkWest Energy Partners, L.P.
4.2	**	—	Form of Subordinated Indenture of MarkWest Energy Partners, L.P.
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. relating to tax matters
12.1	**	—	Calculation of ratios of earnings to fixed charges
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
24.1	**	—	Power of Attorney (included on signature page of this registration statement)
25.1	**	—	Form T-1 Statement of Eligibility and Qualification under the Trust Indenture Act of 1939 of the Trustee under the Senior Indenture
25.2	**	—	Form T-1 Statement of Eligibility and Qualification under the Trust Indenture Act of 1939 of the Trustee under the Subordinated Indenture

(1)

Incorporated by reference to the Form S-1 Registration Statement, filed January 31, 2002.

(2)

Incorporated by reference to Amendment No. 1 to Form S-1, filed March 22, 2002.

(3)

Incorporated by reference to Amendment No. 3 to Form S-1, filed April 24, 2002.

(4)

Incorporated by reference to Amendment No. 5 to Form S-1, filed May 8, 2002.

*

To be filed as an Exhibit to a Current Report on Form 8-K or in a post-effective amendment to this registration statement.

**

Filed herewith.

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Item 17. Undertakings.

        The undersigned registrant hereby undertakes:

        (1)   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;

         (ii)  To reflect in the prospectus any facts or events arising after the effective date of the 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 the 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 prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than 20% change in the maximum aggregate offering price set forth in the "Calculation of Registration Fee" table in the effective registration statement;

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

provided, however, that paragraphs (1)(i) and (1)(ii) do not apply if information required to be included in a post-effective amendment by those paragraphs is contained in periodic reports filed by registrants pursuant to Section 13 or Section 15(d) of the Securities Exchange Act of 1934 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.

        (2)   That, for the purpose of determining any liability under the Securities Act of 1933, 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.

        (3)   To 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.

        (4)   That, for the purpose of determining liability under the Securities Act of 1933 to any purchaser:

          (i)  If the registrant is relying on Rule 430B:

        A. Each prospectus filed by the registrant pursuant to Rule 424(b)(3)shall be deemed to be part of the registration statement as of the date the filed prospectus was deemed part of and included in the registration statement; and

        B. Each prospectus required to be filed pursuant to Rule 424(b)(2), (b)(5), or (b)(7) as part of a registration statement in reliance on Rule 430B relating to an offering made pursuant to Rule 415(a)(1)(i), (vii), or (x) for the purpose of providing the information required by section 10(a) of the Securities Act of 1933 shall be deemed to be part of and included in the registration statement as of the earlier of the date such form of prospectus is first used after effectiveness or the date of the first contract of sale of securities in the offering described in the prospectus. As provided in Rule 430B, for liability purposes of the issuer and any person that is at that date an underwriter, such date shall be deemed to be a new effective date of the registration statement relating to the securities in the registration statement to which that prospectus relates, and the offering of such securities at that time shall be deemed to be the

II-3

initial bona fide offering thereof. Provided, however, that no statement made in a registration statement or prospectus that is part of the registration statement or made in a document incorporated or deemed incorporated by reference into the registration statement or prospectus that is part of the registration statement will, as to a purchaser with a time of contract of sale prior to such effective date, supersede or modify any statement that was made in the registration statement or prospectus that was part of the registration statement or made in any such document immediately prior to such effective date; or

         (ii)  If the registrant is subject to Rule 430C, each prospectus filed pursuant to Rule 424(b) as part of a registration statement relating to an offering, other than registration statements relying on Rule 430B or other than prospectuses filed in reliance on Rule 430A, shall be deemed to be part of and included in the registration statement as of the date it is first used after effectiveness. Provided, however, that no statement made in a registration statement or prospectus that is part of the registration statement or made in a document incorporated or deemed incorporated by reference into the registration statement or prospectus that is part of the registration statement will, as to a purchaser with a time of contract of sale prior to such first use, supersede or modify any statement that was made in the registration statement or prospectus that was part of the registration statement or made in any such document immediately prior to such date of first use.

        (5)   That, for the purpose of determining liability of the registrant under the Securities Act of 1933 to any purchaser in the initial distribution of the securities, the registrant undertakes that in a primary offering of securities of the registrant pursuant to this registration statement, regardless of the underwriting method used to sell the securities to the purchaser, if the securities are offered or sold to such purchaser by means of any of the following communications, the registrant will be a seller to the purchaser and will be considered to offer or sell such securities to such purchaser:

          (i)  Any preliminary prospectus or prospectus of the registrant relating to the offering required to be filed pursuant to Rule 424;

         (ii)  Any free writing prospectus relating to the offering prepared by or on behalf of an undersigned registrant or used or referred to by the registrant;

        (iii)  The portion of any other free writing prospectus relating to the offering containing material information about the registrant or its securities provided by or on behalf of the registrant; and

        (iv)  Any other communication that is an offer in the offering made by the registrant to the purchaser.

        (6)   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 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 the 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.

        (7)   Insofar as indemnification for liabilities arising under the Securities Act of 1933 may be permitted to directors, officers and controlling persons of each Registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the SEC such indemnification is against public policy as expressed in the Securities Act of 1933 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

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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 of 1933 and will be governed by the final adjudication of such issue.

        (8)   The undersigned registrant hereby undertakes to file an application for the purpose of determining the eligibility of the trustee to act under subsection (a) of Section 310 of the Trust Indenture Act ("Act") in accordance with the rules and regulations prescribed by the Commission under Section 305(b)2 of the Act.

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SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and 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 the 15th day of November, 2006.

MARKWEST ENERGY PARTNERS, L.P.
By:	MARKWEST ENERGY GP, L.L.C., its General Partner
By:	/s/  ANDREW L. SCHROEDER
Name:	Andrew L. Schroeder
Title:	Vice President and Treasurer

        KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below constitutes and appoints Nancy K. Buese and Andrew L. Schroeder, and each of them, his true and lawful attorneys-in-fact and agents, with full power of substitution, for him and in his name, place and stead, in any and all capacities, to sign any and all amendments (including post-effective amendments) to this registration statement and any additional registration statement pursuant to Rule 462(b), and to file the same with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorneys-in-fact and agents full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith, as fully and to all intents and purposes as they might or could not in person, hereby ratifying and confirming all that said attorneys-in-fact and agents or any of them, or their or his or her substitute or substitutes may lawfully do or cause to be done by virtue hereof.

        Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement on Form S-3 has been signed below by the following persons in the capacities indicated on the 15th day of November, 2006.

Signature	Title
/s/  FRANK M. SEMPLE Frank M. Semple	President, Chief Executive Officer and Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P. (Principal Executive Officer)
/s/  NANCY K. BUESE Nancy K. Buese	Chief Financial Officer of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P. (Principal Financial Officer and Principal Accounting Officer)
/s/  JOHN M. FOX John M. Fox	Chairman of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.
/s/  DONALD C. HEPPERMANN Donald C. Heppermann	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.

II-6

/s/  CHARLES K. DEMPSTER Charles K. Dempster	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.
/s/  WILLIAM A. KELLSTROM William A. Kellstrom	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.
/s/  WILLIAM P. NICOLETTI William P. Nicoletti	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.
/s/  KEITH E. BAILEY Keith E. Bailey	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.

II-7

SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and 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 the 15th day of November, 2006.

MARKWEST ENERGY FINANCE CORPORATION
By:	/s/  ANDREW L. SCHROEDER
Name:	Andrew L. Schroeder
Title:	Vice President and Treasurer

        KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below constitutes and appoints Nancy K. Buese and Andrew L. Schroeder, and each of them, his true and lawful attorneys-in-fact and agents, with full power of substitution, for him and in his name, place and stead, in any and all capacities, to sign any and all amendments (including post-effective amendments) to this registration statement and any additional registration statement pursuant to Rule 462(b), and to file the same with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorneys-in-fact and agents full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith, as fully and to all intents and purposes as they might or could not in person, hereby ratifying and confirming all that said attorneys-in-fact and agents or any of them, or their or his or her substitute or substitutes may lawfully do or cause to be done by virtue hereof.

        Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement on Form S-3 has been signed below by the following persons in the capacities indicated on the 15th day of November, 2006.

Signature	Title
/s/  FRANK M. SEMPLE Frank M. Semple	Chairman, Chief Executive Officer, President and Director of MarkWest Energy Finance Corporation (Principal Executive Officer)
/s/  NANCY K. BUESE Nancy K. Buese	Chief Financial Officer of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P. (Principal Financial Officer and Principal Accounting Officer)
/s/  ANDREW L. SCHROEDER Andrew L. Schroeder	Vice President, Treasurer and Director of MarkWest Energy Finance Corporation

II-8

SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and 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 the 15th day of November, 2006.

MARKWEST ENERGY OPERATING COMPANY, L.L.C.
By:	MARKWEST ENERGY PARTNERS, L.P., its Managing Member
By:	MARKWEST ENERGY GP, L.L.C., its General Partner
By:	/s/  ANDREW L. SCHROEDER
Name:	Andrew L. Schroeder
Title:	Vice President and Treasurer

        KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below constitutes and appoints Nancy K. Buese and Andrew L. Schroeder, and each of them, his true and lawful attorneys-in-fact and agents, with full power of substitution, for him and in his name, place and stead, in any and all capacities, to sign any and all amendments (including post-effective amendments) to this registration statement and any additional registration statement pursuant to Rule 462(b), and to file the same with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorneys-in-fact and agents full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith, as fully and to all intents and purposes as they might or could not in person, hereby ratifying and confirming all that said attorneys-in-fact and agents or any of them, or their or his or her substitute or substitutes may lawfully do or cause to be done by virtue hereof.

        Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement on Form S-3 has been signed below by the following persons in the capacities indicated on the 15th day of November, 2006.

Signature	Title
/s/  FRANK M. SEMPLE Frank M. Semple	President, Chief Executive Officer and Director of MarkWest Energy GP, L.L.C.,† General Partner of MarkWest Energy Partners, L.P. (Principal Executive Officer)
/s/  NANCY K. BUESE Nancy K. Buese	Chief Financial Officer of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P. (Principal Financial Officer and Principal Accounting Officer)
/s/  JOHN M. FOX John M. Fox	Chairman of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P.

II-9

/s/  DONALD C. HEPPERMANN Donald C. Heppermann	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P.
/s/  CHARLES K. DEMPSTER Charles K. Dempster	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P.
/s/  WILLIAM A. KELLSTROM William A. Kellstrom	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P.
/s/  WILLIAM P. NICOLETTI William P. Nicoletti	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P.
/s/  KEITH E. BAILEY Keith E. Bailey	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.

†

MarkWest Energy GP, L.L.C. is the general partner of MarkWest Energy Partners, L.P., which is the managing member of MarkWest Energy Operating Company, L.L.C.

II-10

SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, as amended, each registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and 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 the 15th day of November, 2006.

BASIN PIPELINE, L.L.C. WEST SHORE PROCESSING COMPANY, L.L.C. MARKWEST ENERGY APPALACHIA, L.L.C. MARKWEST TEXAS GP, L.L.C. MW TEXAS LIMITED, L.L.C. MARKWEST MICHIGAN PIPELINE COMPANY, L.L.C. MARKWEST WESTERN OKLAHOMA GAS COMPANY, L.L.C.
By:	MarkWest Energy Operating Company, L.L.C., its Managing Member
By:	MARKWEST ENERGY PARTNERS, L.P., its Managing Member
By:	MARKWEST ENERGY GP, L.L.C., its General Partner
	/s/  ANDREW L. SCHROEDER Andrew L. Schroeder Vice President and Treasurer

        KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below constitutes and appoints Nancy K. Buese and Andrew L. Schroeder, and each of them, his true and lawful attorneys-in-fact and agents, with full power of substitution, for him and in his name, place and stead, in any and all capacities, to sign any and all amendments (including post-effective amendments) to this registration statement and any additional registration statement pursuant to Rule 462(b), and to file the same with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorneys-in-fact and agents full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith, as fully and to all intents and purposes as they might or could not in person, hereby ratifying and confirming all that said attorneys-in-fact and agents or any of them, or their or his or her substitute or substitutes may lawfully do or cause to be done by virtue hereof.

II-11

        Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement on Form S-3 has been signed below by the following persons in the capacities indicated on the 15th day of November, 2006.

Signature	Title
/s/  FRANK M. SEMPLE Frank M. Semple	President, Chief Executive Officer and Director of MarkWest Energy GP, L.L.C.,† General Partner of MarkWest Energy Partners, L.P. (Principal Executive Officer)
/s/  NANCY K. BUESE Nancy K. Buese	Chief Financial Officer of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P. (Principal Financial Officer and Principal Accounting Officer)
/s/  JOHN M. FOX John M. Fox	Chairman of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P.
/s/  DONALD C. HEPPERMANN Donald C. Heppermann	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P.
/s/  CHARLES K. DEMPSTER Charles K. Dempster	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P.
/s/  WILLIAM A. KELLSTROM William A. Kellstrom	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P.
/s/  WILLIAM P. NICOLETTI William P. Nicoletti	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P.
/s/  KEITH E. BAILEY Keith E. Bailey	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.

†

MarkWest Energy GP, L.L.C. is the general partner of MarkWest Energy Partners, L.P., which is the managing member of MarkWest Energy Operating Company, L.L.C. MarkWest Energy Operating Company, L.L.C. is the managing member of Basin Pipeline L.L.C., West Shore Processing Company, L.L.C., MarkWest Energy Appalachia, L.L.C., MarkWest Texas GP, L.L.C., MW Texas Limited, L.L.C., MarkWest Michigan Pipeline Company, L.L.C. and MarkWest Western Oklahoma Gas Company, L.L.C.

II-12

SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, as amended, each registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and 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 the 15th day of November, 2006.

MARKWEST NEW MEXICO, L.P. MARKWEST PINNACLE L.P. MARKWEST PNG UTILITY L.P. MARKWEST TEXAS PNG UTILITY L.P. MARKWEST BLACKHAWK L.P. MARKWEST POWER TEX L.P. MARKWEST ENERGY EAST TEXAS GAS COMPANY L.P. MARKWEST PIPELINE COMPANY, L.P.
By:	MARKWEST TEXAS GP, L.L.C., its general partner
By:	MarkWest Energy Operating Company, L.L.C., its Managing Member
By:	MARKWEST ENERGY PARTNERS, L.P., its Managing Member
By:	MARKWEST ENERGY GP, L.L.C., its General Partner
By:	/s/  ANDREW L. SCHROEDER
Name:	Andrew L. Schroeder
Title:	Vice President and Treasurer

        KNOW ALL PERSONS BY THESE PRESENTS, that each person whose signature appears below constitutes and appoints Nancy K. Buese and Andrew L. Schroeder, and each of them, his true and lawful attorneys-in-fact and agents, with full power of substitution, for him and in his name, place and stead, in any and all capacities, to sign any and all amendments (including post-effective amendments) to this registration statement and any additional registration statement pursuant to Rule 462(b), and to file the same with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorneys-in-fact and agents full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith, as fully and to all intents and purposes as they might or could not in person, hereby ratifying and confirming all that said attorneys-in-fact and agents or any of them, or their or his or her substitute or substitutes may lawfully do or cause to be done by virtue hereof.

II-13

        Pursuant to the requirements of the Securities Act of 1933, as amended, this Registration Statement on Form S-3 has been signed below by the following persons in the capacities indicated on the 15th day of November, 2006.

Signature	Title
/s/  FRANK M. SEMPLE Frank M. Semple	President, Chief Executive Officer and Director of MarkWest Energy GP, L.L.C.,† General Partner of MarkWest Energy Partners L.P. (Principal Executive Officer)
/s/  NANCY K. BUESE Nancy K. Buese	Chief Financial Officer of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners, L.P. (Principal Financial Officer and Principal Accounting Officer)
/s/  JOHN M. FOX John M. Fox	Chairman of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.
/s/  DONALD C. HEPPERMANN Donald C. Heppermann	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.
/s/  CHARLES K. DEMPSTER Charles K. Dempster	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.
/s/  WILLIAM A. KELLSTROM William A. Kellstrom	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.
/s/  WILLIAM P. NICOLETTI William P. Nicoletti	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.
/s/  KEITH E. BAILEY Keith E. Bailey	Director of MarkWest Energy GP, L.L.C., General Partner of MarkWest Energy Partners L.P.

†

MarkWest Energy GP, L.L.C. is the general partner of MarkWest Energy Partners, L.P., which is the managing member of MarkWest Energy Operating Company, L.L.C. MarkWest Energy Operating Company, L.L.C. is the managing member of MarkWest Texas GP, L.L.C., and MarkWest Texas GP, L.L.C is the general partner of MarkWest New Mexico, L.P., MarkWest Pinnacle L.P., MarkWest PNG Utility L.P., MarkWest Texas PNG Utility L.P., MarkWest Blackhawk L.P., MarkWest Power Tex L.P., MarkWest Pipeline Company, L.P. and MarkWest Energy East Texas Gas Company L.P.

II-14