S-3

As filed with the Securities and Exchange Commission on May 15, 2012

Registration No. 333-            

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-3

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

CLEAN DIESEL TECHNOLOGIES, INC.

(Exact Name of Registrant as Specified in its Charter)

Delaware	3564	06-1393453
(State or Other Jurisdiction of Incorporation or Organization)	(Primary Standard Industrial Classification Code Number)	(I.R.S. Employer Identification No.)

4567 Telephone Road, Suite 100,

Ventura, CA 93003

(805) 639-9460

(Address, Including Zip Code, and Telephone Number, Including Area Code, of Registrant’s Principal Executive Offices)

Nikhil A. Mehta

Chief Financial Officer

Clean Diesel Technologies, Inc.

4567 Telephone Road, Suite 100

Ventura, CA 93003

(805) 639-9467

(Name, Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent for Service)

Copy to:

Steven D. Pidgeon

DLA Piper LLP (US)

2525 East Camelback Road, Suite 1000

Phoenix, AZ 85016-4232

480-606-5124

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:    ¨

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 reinvestment plans, check the following box:    x

If this form is filed to register additional securities for an offering pursuant to Rule 462(b) 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:    ¨

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:    ¨

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:    ¨

Indicate by checkmark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer or a smaller reporting company (as defined in Rule 12b-2 of the Exchange Act).

Large Accelerated Filer	¨	Accelerated Filer	¨
Non-Accelerated Filer	¨	Smaller reporting company	x

CALCULATION OF REGISTRATION FEE

Title of Each Class of Securities to be Registered	To be Registered (1)	Proposed Maximum Offering Price Per Share (1)	Proposed Maximum Aggregate Offering Price (1)(2)	Amount of Registration Fee
Common Stock, par value $0.01 per share (3)
Preferred Stock, par value $0.01 per share(4)
Warrants (5)
Units (6)
Total			$50,000,000	$5,730.00

(1) There are being registered an indeterminate number of securities as shall have an aggregate offering price not to exceed $50,000,000. The securities registered hereunder may be sold separately or with other securities registered hereunder. The proposed maximum aggregate offering price per class of security will be determined from time to time by the registrant in connection with the issuance by the registrant of the securities registered under this registration statement and is not specified as to each class of security being registered under this registration statement pursuant to General Instruction II.D. of Form S-3 under the Securities Act of 1933, as amended (the “Securities Act”). In addition, pursuant to Rule 416 of the rules and regulations under the Securities Act, the securities being registered hereunder include such indeterminate number of shares of common stock and preferred stock as may be issuable with respect to the securities being registered hereunder as a result of stock splits, stock dividends or similar transactions. Separate consideration may or may not be received for securities that are issuable on exercise, conversion or exchange of other securities or that are issued in units.

(2) The proposed maximum offering price has been estimated solely for the purpose of calculating the registration fee in accordance with Rule 457(o) under the Securities Act.

(3) Subject to note 1 above, there is being registered hereunder an indeterminate number of shares of common as may be sold from time to time, at indeterminate prices, by the registrant.

(4) Subject to note 1 above, there is being registered hereunder an indeterminate number of shares of preferred stock (with accompanying purchase rights, if any) as may be sold from time to time, at indeterminate prices, by the registrant. In addition, the securities being registered in this registration statement also consist of such an indeterminate amount of common stock (with accompanying purchase rights, if any) (i) as may be issuable or deliverable upon conversion of shares of preferred stock, and (ii) as may be required for delivery upon conversion of shares of preferred stock as a result of anti-dilution provisions.

(5) Subject to note 1 above, there is being registered hereunder an indeterminate number of warrants to purchase common stock or preferred stock of one or more series registered under this registration statement as may be sold from time to time, at indeterminate prices, by the registrant. In addition, the securities being registered in this registration statement also consist of such an indeterminate amount of common stock and preferred stock (in each case, with accompanying purchase rights, if any) (i) as may be issuable or deliverable upon exercise of warrants and (ii) as may be required for delivery upon exercise of any warrants as a result of anti-dilution provisions.

(6) Subject to note 1 above, there is being registered hereunder an indeterminate number of units , consisting of one or more shares of common stock, shares of preferred stock, warrants, or any combination of such securities, as may be sold from time to time, at indeterminate prices, by the registrant.

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

The information in this prospectus is not complete and may be changed. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities and is not soliciting an offer to buy these securities in any state where the offer or sale is not permitted.

SUBJECT TO COMPLETION, DATED MAY 15, 2012

PROSPECTUS

CLEAN DIESEL TECHNOLOGIES, INC.

$50,000,000

Common Stock

Preferred Stock

Warrants

Units

By this prospectus, we may offer and sell from time to time up to $50,000,000 in total of any combination of the securities described in this prospectus, either individually or in units. We may also offer common stock upon conversion of preferred stock or common stock or preferred stock upon the exercise of warrants. This prospectus provides a general description of securities we may offer. Each time we offer securities, we will provide a prospectus supplement containing more information about the particular offering together with this prospectus. The prospectus supplement also may add, update or change information contained in this prospectus. This prospectus may not be used to offer and sell securities without a prospectus supplement.

The securities may be sold directly by us to investors, through agents designated from time to time or to or through underwriters or dealers, on a delayed or continuous basis. For additional information on the methods of sale, you should refer to the section entitled “Plan of Distribution” in this prospectus. If any agents or underwriters are involved in the sale of any securities, the names of such agents or underwriters and any applicable fees, commissions, discounts and over-allotment options will be set forth in the applicable prospectus supplement.

Our common stock is listed on the NASDAQ Capital Market under the symbol “CDTI.” On May 11, 2012, the last reported sale price of our common stock was $2.57 per share. Pursuant to General Instruction I.B.6 of Form S-3, as long as the aggregate market value of our common stock held by non-affiliates remains below $75.0 million, we will not, during any 12 calendar month period, sell the securities in a public primary offering with a value exceeding more than one-third of the aggregate market value of our common stock held by non-affiliates. As of March 31, 2012, the aggregate market value of our outstanding common stock held by non-affiliates was approximately $14,420,866, which was calculated based on 5,611,232 shares of outstanding common stock held by non-affiliates and a price per share of $2.57, the last reported sale price per share of our common stock on the NASDAQ Capital Market on May 11, 2012. As of the date hereof, we have not offered any securities pursuant to General Instruction I.B.6 of Form S-3 during the prior 12 calendar month period that ends on and includes the date hereof.

Investing in our securities involves a high degree of risk. We refer you to the section entitled “Risk Factors” on page 2 of this prospectus and in the applicable prospectus supplement and under similar sections in the documents we incorporate by reference into this prospectus.

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

The date of this prospectus is             , 2012

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS	1
ABOUT OUR COMPANY	1
RISK FACTORS	2
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS	3
USE OF PROCEEDS	3
DILUTION	3
RATIO OF EARNINGS TO FIXED CHARGES	4
DESCRIPTION OF COMMON STOCK	4
DESCRIPTION OF PREFERRED STOCK	6
DESCRIPTION OF WARRANTS	8
DESCRIPTION OF UNITS	10
PLAN OF DISTRIBUTION	11
LEGAL MATTERS	14
EXPERTS	15
WHERE YOU CAN FIND MORE INFORMATION	15
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE	15

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ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission (“SEC”) using a “shelf” registration process. Under this shelf registration process, we may offer to sell any one or more or a combination of the securities described in this prospectus in one or more offerings up to a total dollar amount of $50,000,000 (or its equivalent based on the applicable exchange rate at the time of the sale in one or more foreign currencies, currency units or composite currencies that we may designate). We have provided to you in this prospectus a general description of the securities we may offer. Each time we sell securities, we will provide a prospectus supplement that will contain specific information about the terms of that offering. We also may add, update or change in the prospectus supplement any of the information contained in this prospectus. If there is an inconsistency between the information in this prospectus and a prospectus supplement, you should rely on the information in the prospectus supplement. You should read carefully both this prospectus and the applicable prospectus supplement together with the documents we incorporate by reference into this prospectus as described under the heading “Incorporation of Certain Documents By Reference” before making an investment decision. THIS PROSPECTUS MAY NOT BE USED TO OFFER AND SELL SECURITIES WITHOUT A PROSPECTUS SUPPLEMENT.

The registration statement that contains this prospectus, including the exhibits to the registration statement and the information incorporated by reference, provides additional information about the securities offered under this prospectus. That registration statement can be read at the SEC web site or at the SEC public reference room as discussed under the heading “Where You Can Find More Information.”

You should rely only on the information provided in this prospectus and in any prospectus supplement, including the information incorporated by reference. We have not authorized anyone to provide you with different information. You should not assume that the information in this prospectus or any supplement to this prospectus is accurate at any date other than the date indicated on the cover page of these documents. We are not making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted.

In this prospectus, references to “Clean Diesel Technologies,” “Clean Diesel,” “CDTI,” the “company,” “we,” “our” or “us,” unless the context otherwise requires, refer to Clean Diesel Technologies, Inc.

ABOUT OUR COMPANY

We are a Delaware corporation formed in 1994 as a wholly-owned subsidiary of Fuel Tech, Inc., a Delaware corporation (formerly known as Fuel-Tech N.V., a Netherlands Antilles limited liability company) (“Fuel Tech”), and were spun off by Fuel Tech in a rights offering in December 1995. Since inception, and as set forth below, we have developed a substantial portfolio of patents and related proprietary rights and extensive technological know-how.

We currently conduct our operations primarily through our wholly-owned subsidiary, Catalytic Solutions, Inc. (“CSI”). CSI is a California corporation formed in 1996 and, through its Heavy Duty Diesel Systems division, has over 30 years of experience in the heavy duty diesel systems market and has proven technical and manufacturing competence in the light duty vehicle catalyst market meeting auto makers’ most stringent requirements. From November 22, 2006 through the Merger, CSI’s common stock was listed on the AIM of the London Stock Exchange (AIM: CTS and CTSU).

We completed a business combination with CSI on October 15, 2010 when our wholly-owned subsidiary, CDTI Merger Sub, Inc., merged with and into CSI. We refer to this transaction as the “Merger.” The Merger was accounted for as a reverse acquisition and, as a result, our company’s (the legal acquirer) consolidated financial statements are now those of CSI (the accounting acquirer), with the assets and liabilities and revenues and expenses of CDTI being included effective from October 15, 2010, the date of the closing of the Merger.

We are headquartered in Ventura, California and have operations in the United States, Canada, the United Kingdom, France, Japan and Sweden as well as an Asian investment. Our proprietary Catalyst division products are manufactured at our facility in Oxnard, California, while our Heavy Duty Diesel Systems division products are manufactured at our facilities in Reno, Nevada; Thornhill, Canada; Malmö, Sweden; and South Godstone, United Kingdom.

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We are a leading global manufacturer and distributor of heavy duty diesel and light duty vehicle emissions control systems and products to major automakers and retrofitters. Our business is driven by increasingly stringent global emission standards for internal combustion engines, which are major sources of a variety of harmful pollutants.

Our Divisions

We operate in two primary divisions: our Heavy Duty Diesel Systems division and our Catalyst division. We have included all of the operations of CDTI in our Heavy Duty Diesel Systems division.

• Heavy Duty Diesel Systems: Our Heavy Duty Diesel Systems division specializes in the design and manufacture of verified exhaust emissions control solutions. This division offers a full range of products for the verified retrofit and original equipment manufacturer, or OEM, markets through its distribution/dealer network and direct sales. These Engine Control Systems (“ECS”) and Clean Diesel Technologies-brand products, such as Purifilter^®, Purifier™, Platinum Plus^® and ARIS^®, along with our exhaust gas recirculation with selective catalytic reduction technologies, are used to reduce exhaust emissions created by on-road, off-road and stationary diesel and alternative fuel engines including propane and natural gas.

Sales of emission control systems by our Heavy Duty Diesel Systems division are being driven by increased regulation of diesel emissions, particularly in the State of California and Europe’s Low Emission Zones, or LEZs. The U.S. Environmental Protection Agency, or EPA, estimated in a 2010 report that more than 11 million diesel engines operating today do not meet its new clean diesel standards, yet the engines can operate for 20 to 30 years. In California, government mandates could lead to the long-term retrofitting of nearly one million diesel vehicles at an estimated cost of over $2 billion, according to a California Air Resources Board, or CARB, 2010 report. According to 2011 data received from Transport for London, we believe the London LEZ regulations will have resulted in the retrofitting of up to 20,000 heavy duty diesel vehicles during 2011 and through early 2012.

• Catalyst: Our Catalyst division produces catalyst formulations to reduce emissions from gasoline, diesel and natural gas combustion engines. Using our proprietary MPC^® technology, we have developed a family of unique high-performance catalysts — with base-metals or low platinum group metal and zero-platinum group metal content — to provide increased catalytic function and value for technology-driven automotive industry customers. Our technical and manufacturing competence in the light duty vehicle market is aimed at meeting auto makers’ most stringent requirements, and we have supplied over ten million parts to light duty vehicle customers since 1996. Our Catalyst division also provides catalyst formulations for our Heavy Duty Diesel Systems division.

Globally, the catalyst market is estimated to exceed $7 billion by 2015, according to a report issued by Global Industry Analysts, Inc. in 2011. We expect growth in this business division to be driven by increased sales to existing customers, including Honda, and catalyst sales internally to our Heavy Duty Diesel Systems division.

With over 30 years experience in vehicle emissions control technologies, we believe we offer one of the industry’s most comprehensive portfolios of evaluated and EPA- and CARB-verified systems for use in engine retrofit programs, as well as by regulators in several European countries.

RISK FACTORS

An investment in our securities involves a high degree of risk. You should carefully consider the risk factors described in Part I, Item 1A, “Risk Factors” in our Annual Report on Form 10-K for the fiscal year ended December 31, 2011, as well as the risks, uncertainties and additional information set forth in the other documents

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incorporated by reference in this prospectus. For a description of these reports and documents, and information about where you can find them, see “Where You Can Find More Information” and “Incorporation of Certain Documents by Reference.” Before making any investment decision, you should carefully consider these risks as well as other information we include or incorporate by reference in this prospectus or in any applicable prospectus supplement. The risks and uncertainties described in the prospectus supplement and the documents we incorporate by reference into this prospectus are not the only ones we face. Additional risks and uncertainties that we are unaware of or that we believe are not material at the time could also materially adversely affect our business, financial condition or results of operations. In any case, the value of our securities could decline, and you could lose all or part of your investment. See also the information contained under the heading “Cautionary Statement Regarding Forward-Looking Statements” immediately below.

CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and the information incorporated by reference in this prospectus include “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933, as amended, (the “Securities Act”), and Section 21E of the Securities Exchange Act of 1934, as amended (the “Exchange Act”). Without limiting the foregoing, the words “may,” “believe,” “may,” “could,” “might,” “possible,” “potential,” “project,” “will,” “should,” “expect,” “intend,” “plan,” “predict,” “anticipate,” “estimate,” “approximate,” “contemplate” or “continue” “target,” “goal” and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these words. All forward-looking statements included in this prospectus are based on information available to us up to, and including, the date of this document, and we assume no obligation to update any such forward-looking statements to reflect events or circumstances that arise after the date hereof. Our actual results could differ materially from those anticipated in these forward-looking statements as a result of certain important factors, including those contained in or incorporated by reference into this prospectus. You should carefully review those factors and also carefully review the risks outlined in other documents that we file from time to time with the SEC.

USE OF PROCEEDS

We will retain broad discretion over the use of net proceeds to us from the sale of our securities offered hereby. Except as may be otherwise described in a prospectus supplement, we currently anticipate using any net proceeds to us for general corporate purposes, which may include working capital, general and administrative expenses, and capital expenditures. We may also use a portion of the net proceeds to acquire or invest in businesses, products and technologies that are complementary to our current business, although we have no present commitments or agreements for any such transactions. The amounts and timing of our actual expenditures for each purpose may vary significantly depending upon numerous factors, including the actual amount of proceeds we receive, competition and economic or other conditions.

We cannot guarantee that we will receive any proceeds in connection with any offering hereunder because we may choose not to issue any of the securities covered by this prospectus. If we choose to issue securities, we will provide a prospectus supplement that will contain information about how we intend to use the proceeds from each such offering.

Pending the application of such proceeds, we may invest the proceeds in short-term, interest bearing, investment-grade marketable securities or money market obligations.

DILUTION

If there is a material dilution of the purchasers’ equity interest from the sale of common equity securities offered under this prospectus, we will set forth in any prospectus supplement the following information regarding any such material dilution:

• the net tangible book value per share of our equity securities before and after the offering;

• the amount of the increase in such net tangible book value per share attributable to the cash payments made by the purchasers in the offering; and

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• the amount of the immediate dilution from the public offering price which will be absorbed by such purchasers.

RATIO OF EARNINGS TO FIXED CHARGES

Because we have no preferred stock issued (and have not had any issued during the last five fiscal years), a ratio of earnings to combined fixed charges and preferred dividends is not presented. If we issue preferred stock in the future, we will set forth in any prospectus supplement the ratio of earnings to combined fixed charges and preferred dividends for the last five fiscal years.

DESCRIPTION OF COMMON STOCK

The following description of our common stock, together with the additional information we include in any applicable prospectus supplement, summarizes the material terms and provisions of our common stock that we may offer under this prospectus. For the complete terms of our common stock, please refer to our Amended and Restated Certificate of Incorporation, as amended (“Certificate of Incorporation”) and Bylaws. The description herein does not contain all of the information that you may find useful or that may be important to you. You should refer to the provisions of our Certificate of Incorporation and Bylaws because they, and not the summary, define the rights of holders of shares of our common stock. You can obtain copies of our Certificate of Incorporation and Bylaws by following the directions under the heading “Where You Can Find More Information.” The terms of our common stock also may be affected by Delaware law.

Authorized and Outstanding Capital Stock

We are authorized to issue 12,000,000 shares of common stock, $0.01 par value per share. As of May 11, 2012, we had 7,218,807 shares of common stock outstanding. In addition, we had 934,914 shares of common stock reserved for issuance under outstanding warrants; 1,783,330 shares reserved for issuance to Lincoln Park Capital Fund, LLC under a purchase agreement; 369,853 shares reserved for issuance to Kanis S.A. upon conversion of our subordinated convertible notes; 176,676 shares reserved for issuance upon the exercise of outstanding stock options and an additional 58,892 shares reserved for issuance pertaining to outstanding restricted stock units under inducement awards; and 577,214 shares reserved for issuance upon the exercise of outstanding stock options and an additional 119,225 shares reserved for issuance pertaining to outstanding restricted stock units under our 1994 Incentive Plan. As of the date of this prospectus, we do not have any shares of preferred stock outstanding.

Voting Rights

For all matters submitted to a vote of stockholders, each holder of common stock is entitled to one vote for each share registered in the holder’s name on our books. Our common stock does not have cumulative voting rights. The holders of a plurality of the shares of our common stock entitled to vote in any election of directors, voting together as a single class, can elect all of the directors standing for election, if they so choose.

Dividends

Subject to limitations under Delaware law and preferences that may be applicable to any then outstanding preferred stock, holders of common stock are entitled to receive ratably those dividends, if any, as may be declared by our Board of Directors out of legally available funds. We have never paid any cash dividends on our Common Stock and do not anticipate paying dividends in the near future.

Liquidation

Upon our liquidation, dissolution or winding up, the holders of common stock will be entitled to share ratably in the net assets legally available for distribution to stockholders after the payment of all of our debts and other liabilities of our company, subject to the prior rights of any preferred stock then outstanding.

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Fully Paid and Nonassessable

All shares of our outstanding common stock are fully paid and nonassessable and any additional shares of common stock that we issue will be fully paid and nonassessable.

Anti-Takeover Effects of Certain Charter and Bylaw Provisions and Delaware Law

Our Certificate of Incorporation and Bylaws and Delaware law contain provisions that could make the following transactions more difficult: acquisition of us by means of a tender offer; acquisition of us by means of a proxy contest or otherwise; or removal of our incumbent officers and directors. It is possible that these provisions could make it more difficult to accomplish or could deter transactions that stockholders may otherwise consider to be in their best interest or in our best interests, including transactions that might result in a premium over the market price for our shares.

The provisions of our Certificate of Incorporation and Bylaws and Delaware law summarized below may have the effect of deterring hostile takeovers or delaying changes in control or management of us. They are designed to encourage persons seeking to acquire control of us to first negotiate with our Board of Directors. We believe that the benefits of increased protection of our potential ability to negotiate with the proponent of an unfriendly or unsolicited proposal to acquire or restructure us outweigh the disadvantages of discouraging these proposals because negotiation of these proposals could result in an improvement of their terms.

Charter and Bylaw Provisions

Our Certificate of Incorporation and Bylaws contain provisions relating to corporate governance and to the rights of stockholders. Our Bylaws provide that special meetings of stockholders may only be called by our Board of Directors, our Chairman of the Board or our Chief Executive Officer and shall be called by our Chairman, Chief Executive Officer or Secretary at the request in writing of stockholders owning at least one-fourth of the outstanding shares of capital stock entitled to vote. In addition, our Certificate of Incorporation provides that our Board of Directors may authorize the issuance of preferred stock without further stockholder approval and upon those terms and conditions, and having those rights, privileges and preferences, as our Board of Directors may determine.

Since the terms of our Certificate of Incorporation and Bylaws may differ from the general information we are providing, you should only rely on the actual provisions of our Certificate of Incorporation and Bylaws. If you would like to read our Certificate of Incorporation and Bylaws, they are available as described under the heading “Where You Can Find More Information.”

Delaware Anti-Takeover Law

We are subject to the provisions of Section 203 of the Delaware General Corporation Law. (“DGCL”). In general, this section prohibits a publicly held Delaware corporation from engaging in a business combination with an interested stockholder for a period of three years after the date of the transaction in which the person becomes an interested stockholder, unless:

• before the date on which the stockholder became an interested stockholder, the corporation’s Board of Directors approved either the business combination or the transaction in which the person became an interested stockholder;

• the stockholder acquires more than 85% of the outstanding voting stock of the corporation, excluding shares held by directors who are officers or held in certain employee stock plans, upon consummation of the transaction in which the stockholder becomes an interested stockholder; or

• the business combination is approved by the Board of Directors and by two-thirds of the outstanding voting stock of the corporation that is not held by the interested stockholder, at a meeting of the stockholders held on or after the date of the business combination.

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Section 203 defines “business combination” to include:

• any merger or consolidation involving the corporation and the interested stockholder;

• any sale, transfer, pledge or other disposition of 10% or more of our assets involving the interested stockholder;

• in general, any transaction that results in the issuance or transfer by us of any of our stock to the interested stockholder; or

• the receipt by the interested stockholder of the benefit of any loans, advances, guarantees, pledges or other financial benefits provided by or through the corporation.

An interested stockholder is a person who, together with affiliates and associates, owns, or at any time within the prior three years did own, 15% or more of the corporation’s voting stock. Business combinations include, without limitation, mergers, consolidations, stock sales, asset sales or other transactions resulting in a financial benefit to interested stockholders.

Other Rights and Restrictions

Holders of our common stock do not have preemptive or subscription rights, and they have no right to convert their common stock into any other securities. There are no redemption or sinking fund provisions applicable to the common stock. The rights, preferences and privileges of common stockholders are subject to the rights of the stockholders of any series of preferred stock which we may designate in the future. Our Certificate of Incorporation and Bylaws do not restrict the ability of a holder of common stock to transfer the holder’s shares of common stock.

Listing

Our common stock is listed on the NASDAQ Capital Market under the symbol “CDTI.”

Transfer Agent and Registrar

The transfer agent and registrar for our common stock is American Stock Transfer & Trust Company, LLC.

DESCRIPTION OF PREFERRED STOCK

The following description of our preferred stock, together with the additional information we include in any applicable prospectus supplement, summarizes the material terms and provisions of our preferred stock that we may offer under this prospectus. For the complete terms of our preferred stock, please refer to our Certificate of Incorporation and Bylaws, and the applicable certificate of designation to our Certificate of Incorporation (“Certificate of Designation”) establishing the terms of the related series of preferred stock. See “Where You Can Find More Information.” The descriptions herein and in the applicable prospectus supplement do not contain all of the information that you may find useful or that may be important to you. You should refer to the provisions of our Certificate of Incorporation, the applicable Certificate of Designation and our Bylaws because they, and not the summaries, define your rights as holders of shares of our preferred stock. The terms of our preferred stock also may be affected by Delaware law.

Authorized and Outstanding Shares

We currently have authorized 100,000 shares of preferred stock, $0.01 par value per share, none of which have been issued. The Board of Directors has the right, without shareholder approval, to issue preferred stock and to establish its terms and conditions.

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Designations, Powers, Preferences, Rights, Qualifications, Limitations and Restrictions

Prior to issuance of shares of each series of our undesignated preferred stock, our Board of Directors is required by the DGCL and our Certificate of Incorporation to adopt resolutions and file a Certificate of Designations with the Secretary of State of the State of Delaware, fixing for each such series the designations, powers, preferences, rights, qualifications, limitations and restrictions of the shares of such series.

Our Board of Directors could authorize the issuance of shares of preferred stock with terms and conditions which could have the effect of discouraging a takeover or other transaction which holders of some, or a majority, of such shares might believe to be in their best interests or in which holders of some, or a majority, of such shares might receive a premium for their shares over the then-market price of such shares.

Subject to limitations prescribed by the DGCL, our Certificate of Incorporation and our Bylaws, our Board of Directors is authorized to fix the number of shares constituting each series of preferred stock and the designations, powers, preferences, rights, qualifications, limitations and restrictions of the shares of such series, including such provisions as may be desired concerning voting, redemption, dividends, dissolution or the distribution of assets, conversion or exchange, and such other subjects or matters as may be fixed by resolution of the Board of Directors. Each series of preferred stock that we offer under this prospectus will, when issued, be fully paid and nonassessable and will not have, or be subject to, any preemptive or similar rights.

The applicable prospectus supplement will describe the following terms of the series of preferred stock in respect of which this prospectus is being delivered:

• the title and stated value of the preferred stock;

• the number of shares of the preferred stock offered, the liquidation preference per share and the purchase price of the preferred stock;

• the dividend rate(s), period(s) and/or payment date(s) or the method(s) of calculation for dividends;

• whether dividends shall be cumulative or non-cumulative and, if cumulative, the date from which dividends on the preferred stock shall accumulate;

• the procedures for any auction and remarketing, if any, for the preferred stock;

• the provisions for a sinking fund, if any, for the preferred stock;

• the provisions for redemption, if applicable, of the preferred stock;

• any listing of the preferred stock on any securities exchange or market;

• the terms and conditions, if applicable, upon which the preferred stock will be convertible into common stock, including the conversion price (or its manner of calculation) and conversion period;

• voting rights, if any, of the preferred stock;

• a discussion of any material and/or special U.S. federal income tax considerations applicable to the preferred stock;

• whether interests in the preferred stock will be represented by depositary shares;

• the relative ranking and preferences of the preferred stock as to dividend rights and rights upon liquidation, dissolution or winding up of our affairs;

• any limitations on issuance of any class or series of preferred stock ranking senior to or on a parity with the preferred stock as to dividend rights and rights upon liquidation, dissolution or winding up of our affairs; and

• any other specific terms, preferences, rights, limitations or restrictions on the preferred stock.

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Transfer Agent and Registrar

The transfer agent and registrar for our preferred stock will be set forth in the applicable prospectus supplement.

DESCRIPTION OF WARRANTS

The following description, together with the additional information we may include in any applicable prospectus supplement, summarizes the material terms and provisions of the warrants that we may offer under this prospectus and the related warrant agreements and warrant certificates. While the terms summarized below will apply generally to any warrants that we may offer, we will describe the particular terms of any series of warrants in more detail in the applicable prospectus supplement. If we indicate in the prospectus supplement, the terms of any warrants offered under that prospectus supplement may differ from the terms described below.

We will file as exhibits to the registration statement of which this prospectus is a part, or will incorporate by reference from reports that we file with the SEC, the form of warrant agreement, including a form of warrant certificate, that describes the terms of the series of warrants we are offering, and any supplemental agreements, before the issuance of the related series of warrants. The following summaries of material terms and provisions of the warrant agreements and warrant certificate are subject to, and qualified in their entirety by reference to, all the provisions of the warrant agreement and warrant certificate applicable to the particular series of warrants that we may offer under this prospectus. We urge you to read the applicable prospectus supplements related to the particular series of warrants that we may offer under this prospectus and the complete warrant agreements and warrant certificates that contain the terms of the warrants. For more information, please review the forms of these documents, which will be filed with the SEC and will be available as described under the heading “Where You Can Find More Information.”

The following is a summary of our outstanding warrants as of May 11, 2012:

	Shares		Weighted Average Exercise Price		Range of Exercise Prices
Outstanding at December 31, 2011		929,914	$	15.13	$2.80 - $169.47
Warrants issued		5,000	$	3.80	$3.80
Outstanding at May 11, 2012		934,914	$	15.07	$2.80 - $169.47
Warrants exercisable at May 11, 2012		904,914	$	15.26	$2.80 - $169.47
Aggregate intrinsic value	$	—

General

We may issue warrants for the purchase of common stock or preferred stock in one or more series. We may issue warrants independently or together with common stock or preferred stock, and the warrants may be attached to or separate from these securities.

We will evidence each series of warrants by warrant certificates that we will issue under a separate agreement. We may enter into a warrant agreement with a warrant agent. If we elect to do so, the warrant agent will act solely as our agent in connection with the warrants and will not assume any obligation or relationship of agency or trust for or with any registered holders of warrants or beneficial owners of warrants. We will indicate the name and address and other information regarding the warrant agent in the applicable prospectus supplement relating to a particular series of warrants if we elect to use a warrant agent.

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We will describe in the applicable prospectus supplement the terms of the series of warrants, including:

• the offering price and aggregate number of warrants offered;

• the currency or currencies (including composite currencies), and/or the securities (if any), in which the exercise price of the warrants may be payable; and, if the exercise price is payable in whole or in part with securities, the basis for determining the amount or number of such securities to be provided as such payment;

• if applicable, the designation and terms of the securities with which the warrants are issued and the number of warrants issued with each such security or each principal amount of such security;

• if applicable, the date on and after which the warrants and the related securities will be separately transferable;

• the number of shares of common stock or preferred stock, as the case may be, purchasable upon the exercise of one warrant and the price at which these shares may be purchased upon such exercise;

• the effect of any merger, consolidation, sale or other disposition of our business on the warrant agreement and the warrants;

• the terms of any rights to redeem or call the warrants;

• any provisions for changes to or adjustments in the exercise price or number of securities issuable upon exercise of the warrants;

• if applicable, the minimum or maximum amount of such warrants that may be exercised at any one time;

• the dates on which the right to exercise the warrants will commence and expire;

• the manner in which the warrant agreement and warrants may be modified;

• if applicable, material U.S. federal income tax consequences of holding or exercising the warrants;

• the terms of the securities issuable upon exercise of the warrants;

• the identity of the warrant agent for the warrants and of any other depositaries, execution or paying agents, transfer agents, registrars or other agents;

• the proposed listing, if any, of the warrants or any securities purchasable upon exercise of the warrants on any securities exchange;

• the anti-dilution provisions of the warrants, if any;

• any redemption or call provisions; and

• any other specific terms, preferences, rights or limitations of or restrictions on the warrants.

Before exercising their warrants, holders of warrants will not have any of the rights of holders of the securities purchasable upon such exercise, including the right to receive dividends, if any, or, payments upon our liquidation, dissolution or winding up or to exercise voting rights, if any.

Exercise of Warrants

Each warrant will entitle the holder to purchase the securities that we specify in the applicable prospectus supplement at the exercise price that we describe in the applicable prospectus supplement. Unless we otherwise specify in the applicable prospectus supplement, holders of the warrants may exercise the warrants at any time up to 5:00 p.m., Pacific Standard Time, on the expiration date that we set forth in the applicable prospectus supplement. After the close of business on the expiration date, unexercised warrants will become void.

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Holders of warrants may exercise their respective warrants in the manner set forth in the applicable prospectus supplement relating to such warrants. We will set forth on the reverse side of the warrant certificate and in the applicable prospectus supplement the information that the holder of the warrant will be required to deliver to the warrant agent.

Upon receipt of the required payment and the warrant certificate properly completed and duly executed at the corporate trust office of the warrant agent or any other office indicated in the applicable prospectus supplement, we will issue and deliver the securities purchasable upon such exercise. If fewer than all of the warrants represented by the warrant certificate are exercised, then we will issue a new warrant certificate for the remaining amount of warrants. If we so indicate in the applicable prospectus supplement, holders of the warrants may surrender securities as all or part of the exercise price for warrants.

Enforceability of Rights by Holders of Warrants

Each warrant agent will act solely as our agent under the applicable warrant agreement and will not assume any obligation or relationship of agency or trust with any holder of any warrant. A single bank or trust company may act as warrant agent for more than one issue of warrants. A warrant agent will have no duty or responsibility in case of any default by us under the applicable warrant agreement or warrant, including any duty or responsibility to initiate any proceedings at law or otherwise, or to make any demand upon us. Any holder of a warrant may, without the consent of the related warrant agent or the holder of any other warrant, enforce by appropriate legal action its right to exercise, and receive the securities purchasable upon exercise of, its warrants.

DESCRIPTION OF UNITS

The following description, together with the additional information we may include in any applicable prospectus supplements, summarizes the material terms and provisions of the units that we may offer under this prospectus and any related unit agreements and unit certificates. While the terms summarized below will apply generally to any units that we may offer, we will describe the particular terms of any series of units in more detail in the applicable prospectus supplement. If we indicate in the prospectus supplement, the terms of any units offered under that prospectus supplement may differ from the terms described below.

We will file as exhibits to the registration statement of which this prospectus is a part, or will incorporate by reference from reports that we file with the SEC, any form of unit agreement that describes the terms of the series of units we are offering, and any supplemental agreements, before the issuance of the related series of units. The following summaries of material terms and provisions of the units are subject to, and qualified in their entirety by reference to, all the provisions of such unit agreements and any supplemental agreements applicable to a particular series of units. We urge you to read the applicable prospectus supplements related to the particular series of units that we may offer under this prospectus and the complete unit agreement and any supplemental agreements that contain the terms of the units. For more information, please review the forms of these documents, which will be filed with the SEC and will be available as described under the heading “Where You Can Find More Information.”

General

We may issue, in one or more series, units comprised of shares of our common stock or preferred stock and warrants to purchase common stock or preferred stock or any combination. Each unit will be issued so that the holder of the unit is also the holder of each security included in the unit. Thus, the holder of a unit will have the rights and obligations of a holder of each included security. The unit agreement under which a unit is issued may provide that the securities included in the unit may not be held or transferred separately, at any time or at any time before a specified date.

We may evidence units by unit certificates that we issue under a separate agreement. We may issue the units under a unit agreement between us and one or more unit agents. If we elect to enter into a unit agreement with a unit agent, the unit agent will act solely as our agent in connection with the units and will not assume any obligation or relationship of agency or trust for or with any registered holders of units or beneficial owners of units. We will indicate the name and address and other information regarding the unit agent in the applicable prospectus supplement relating to a particular series of units if we elect to use a unit agent.

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We will describe in the applicable prospectus supplement the terms of the series of units being offered, including:

• the designation and terms of the units and of the securities comprising the units, including whether and under what circumstances those securities may be held or transferred separately;

• any provisions of the governing unit agreement that differ from those described below; and any provisions for the issuance, payment, settlement, transfer or exchange of the units or of the securities comprising the units.

The other provisions regarding our common stock, preferred stock and warrants as described in this section will apply to each unit to the extent such unit consists of shares of our common stock and preferred stock and warrants to purchase our common stock.

Enforceability of Rights by Holders of Units

Each unit agent will act solely as our agent under the applicable unit agreement and will not assume any obligation or relationship of agency or trust with any holder of any unit. A single bank or trust company may act as unit agent for more than one series of units. A unit agent will have no duty or responsibility in case of any default by us under the applicable unit agreement or unit, including any duty or responsibility to initiate any proceedings at law or otherwise, or to make any demand upon us. Any holder of a unit may, without the consent of the related unit agent or the holder of any other unit, enforce by appropriate legal action its rights as holder under any security included in the unit.

PLAN OF DISTRIBUTION

We may sell the securities offered by this prospectus in one or more of the following ways from time to time:

• to or through underwriters or dealers; or

• directly to purchasers, including our affiliates, or to a single purchaser;

• through one or more agents;

• through a block trade in which the broker or dealer engaged to handle the block will attempt to sell the securities as agent, but may position and resell a portion of the block as principal to facilitate the transaction; or

• through a combination of any of these methods of sale.

In addition, we may issue the securities being offered by this prospectus as a dividend or distribution.

We may effect the distribution of the securities from time to time in one or more transactions at a fixed price or prices, which may be changed from time to time, at market prices prevailing at the time of sale, at prices related to prevailing market prices or at negotiated prices.

We will set forth in a prospectus supplement the terms of the offering of our securities, including:

• the type and amount of securities we are offering;

• the purchase price of our securities being offered and the net proceeds we will receive from the sale;

• the method of distribution of the securities we are offering;

• the name or names of any agents, underwriters or dealers;

• any over-allotment options under which underwriters may purchase additional securities from us;

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• any underwriting discounts and commissions or agency fees and commissions and other items constituting underwriters’ or agents’ compensation;

• any discounts or concessions allowed or re-allowed or paid to dealers; and

• any securities exchanges on which such securities may be listed.

Sale through Underwriters and Dealers

If we use an underwriter or underwriters in the sale of securities offered by this prospectus, the underwriters will acquire the securities for their own account, including through underwriting, purchase, security lending or repurchase agreements with us. The underwriters may resell the securities from time to time in one or more transactions, including negotiated transactions. Underwriters may sell the securities in order to facilitate transactions in any of our other securities (described in this prospectus or otherwise), including other public or private transactions and short sales.

Underwriters may offer securities to the public either through underwriting syndicates represented by one or more managing underwriters or directly by one or more firms acting as underwriters. Unless otherwise indicated in the prospectus supplement, the obligations of the underwriters to purchase the securities will be subject to certain conditions, and the underwriters will be obligated to purchase all the offered securities if they purchase any of them. The underwriters may change from time to time any public offering price and any discounts or concessions allowed or re-allowed or paid to dealers.

If we use an underwriter or underwriters in the sale of securities, we will execute an underwriting agreement with the underwriter or underwriters at the time we reach an agreement for sale. We will set forth in the applicable prospectus supplement the names of the specific managing underwriter or underwriters, as well as any other underwriters, and the terms of the transactions, including compensation of the underwriters and dealers. This compensation may be in the form of discounts, concessions or commissions.

In compliance with the guidelines of the Financial Industry Regulatory Authority, or FINRA, the aggregate maximum discount, commission, agency fees, or other items constituting underwriting compensation to be received by any FINRA member or independent broker-dealer will not exceed 8% of any offering pursuant to this prospectus and any applicable prospectus supplement.

We may grant to the underwriters options to purchase additional securities to cover over-allotments, if any, at the public offering price with additional underwriting discounts or commissions. If we grant any over-allotment option, the terms of any over-allotment option will be set forth in the prospectus supplement relating to those securities.

Sale Through Dealers

If we use dealers in the sale of the securities offered by this prospectus, we or an underwriter will sell the securities to them as principals. The dealers may then resell those securities to the public at varying prices to be determined by the dealers at the time of resale. The applicable prospectus supplement will set forth the names of the dealers and the terms of the transactions.

Direct Sales

We may directly solicit offers to purchase the securities offered by this prospectus. In this case, no underwriters or agents would be involved. We may sell the securities directly to institutional investors or others who may be deemed to be underwriters within the meaning of the Securities Act with respect to any sale of those securities. The terms of any such sales will be described in the prospectus supplement.

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Sale Through Agents

Securities also may be offered and sold through agents designated from time to time. The prospectus supplement will name any agent involved in the offer or sale of the securities and will describe any commissions payable to the agent. Unless otherwise indicated in the applicable prospectus supplement, any agent will agree to use its reasonable best efforts to solicit purchases for the period of its appointment. Any agent may be deemed to be an underwriter within the meaning of the Securities Act with respect to any sale of those securities.

Delayed Delivery Contracts

If the applicable prospectus supplement indicates, we may authorize agents, underwriters or dealers to solicit offers from institutions to purchase securities at the public offering price under delayed delivery contracts. These contracts would provide for payment and delivery on a specified date in the future. Institutions with which contracts of this type may be made include commercial and savings banks, insurance companies, pension funds, investment companies, educational and charitable institutions, but in all cases those institutions must be approved by us. The obligations of any purchaser under any contract of this type will be subject to the condition that the purchase of the securities shall not at the time of delivery be prohibited under the laws of the jurisdiction to which the purchaser is subject. The applicable prospectus supplement will describe the commission payable for solicitation of those contracts.

Market Making, Stabilization and Other Transactions

Our common stock is listed on the NASDAQ Capital Market. Any common stock sold pursuant to a prospectus supplement will be eligible for listing and trading on the NASDAQ Capital Market, subject to official notice of issuance. Unless the applicable prospectus supplement states otherwise, each other class or series of securities issued will be a new issue and will have no established trading market. We may elect to list any other class or series of securities on an exchange, but we are not currently obligated to do so.

Any underwriters that we use in the sale of offered securities may make a market in such securities, but may discontinue such market making at any time without notice. Therefore, we cannot assure you that the securities will have a liquid trading market. Any underwriter may also engage in stabilizing transactions, syndicate covering transactions and penalty bids in accordance with Regulation M under the Securities Exchange Act of 1934, as amended. Stabilizing transactions involve bids to purchase the underlying security in the open market for the purpose of pegging, fixing or maintaining the price of the securities. Syndicate covering transactions involve purchases of the securities in the open market after the distribution has been completed in order to cover syndicate short positions.

Penalty bids permit the underwriters to reclaim a selling concession from a syndicate member when the securities originally sold by the syndicate member are purchased in a syndicate covering transaction to cover syndicate short positions. Stabilizing transactions, syndicate covering transactions and penalty bids may cause the price of the securities to be higher than it would be in the absence of the transactions. The underwriters may, if they commence these transactions, discontinue them at any time.

The effect of these transactions may be to stabilize or maintain the market price of the securities at a level above that which might otherwise prevail in the open market. Such transactions, if commenced, may be discontinued at any time. We make no representation or prediction as to the direction or magnitude of any effect that the transactions described above, if implemented, may have on the price of our securities.

Derivative Transactions and Hedging

We, the underwriters or other agents may engage in derivative transactions involving the securities. These derivatives may consist of short sale transactions and other hedging activities. The underwriters or agents may acquire a long or short position in the securities, hold or resell securities acquired and purchase options or futures on the securities and other derivative instruments with returns linked to or related to changes in the price of the securities. In order to facilitate these derivative transactions, we may enter into security lending or repurchase agreements with the underwriters or agents. The underwriters or agents may effect the derivative transactions through sales of the securities to the public, including short sales, or by lending the securities in order to facilitate short sale transactions by others.

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The underwriters or agents may also use the securities purchased or borrowed from us or others (or, in the case of derivatives, securities received from us in settlement of those derivatives) to directly or indirectly settle sales of the securities or close out any related open borrowings of the securities.

Electronic Auctions

We also may make sales through the Internet or through other electronic means. Since we may from time to time elect to offer securities directly to the public, with or without the involvement of agents, underwriters or dealers, utilizing the Internet or other forms of electronic bidding or ordering systems for the pricing and allocation of such securities, you will want to pay particular attention to the description of that system we will provide in a prospectus supplement.

Such electronic system may allow bidders to directly participate, through electronic access to an auction site, by submitting conditional offers to buy that are subject to acceptance by us, and which may directly affect the price or other terms and conditions at which such securities are sold. These bidding or ordering systems may present to each bidder, on a so-called “real-time” basis, relevant information to assist in making a bid, such as the clearing spread at which the offering would be sold, based on the bids submitted, and whether a bidder’s individual bids would be accepted, prorated or rejected. Of course, many pricing methods can and may also be used.

Upon completion of such an electronic auction process, securities will be allocated based on prices bid, terms of bid or other factors. The final offering price at which securities would be sold and the allocation of securities among bidders would be based in whole or in part on the results of the Internet or other electronic bidding process or auction.

General Information

Agents, underwriters, and dealers may be entitled, under agreements entered into with us, to indemnification by us against specified liabilities, including liabilities under the Securities Act of 1933, or to contribution by us to payments they may be required to make in respect to such liabilities. The applicable prospectus supplement will describe the terms and conditions of indemnification or contribution. Some of our agents, underwriters, and dealers, or their affiliates, may be customers of, engage in transactions with or perform services for us, in the ordinary course of business. We will describe in the prospectus supplement the nature of any such relationship and the name of the parties involved. Any lockup arrangements will be set forth in the applicable prospectus supplement.

Under Rule 15c6-1 of the Exchange Act, trades in the secondary market generally are required to settle in three business days, unless the parties to any such trade expressly agree otherwise. The applicable prospectus supplement may provide that the original issue date for your securities may be more than three scheduled business days after the trade date for your securities. Accordingly, in such a case, if you wish to trade securities on any date prior to the third business day before the original issue date for your securities, you will be required, by virtue of the fact that your securities initially are expected to settle in more than three scheduled business days after the trade date for your securities, to make alternative settlement arrangements to prevent a failed settlement.

The securities may be new issues of securities and may have no established trading market. The securities may or may not be listed on a national securities exchange. We can make no assurance as to the liquidity of or the existence of trading markets for any of the securities.

LEGAL MATTERS

Unless the applicable prospectus supplement indicates otherwise, certain legal matters with respect to the validity of the securities offered under this prospectus and any prospectus supplement will be passed upon for us by DLA Piper LLP (US), Phoenix, Arizona. Counsel for any underwriter or agent will be noted in the applicable prospectus supplement.

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EXPERTS

The consolidated financial statements of Clean Diesel Technologies, Inc. as of December 31, 2011 and 2010, and for the years 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 such firm as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC. You may read and copy any document we file at the SEC’s Public Reference Room at 100 F Street, N.E., Room 1580, Washington, D.C. 20549. You can request copies of these documents by writing to the SEC and paying a fee for the copying cost. Please call the SEC at 1-800-SEC-0330 for more information about the operation of the public reference room. Our SEC filings are also available to the public at the SEC’s web site at http://www.sec.gov.

We have filed with the SEC a registration statement on Form S-3 under the Securities Act relating to the securities covered by this prospectus. The registration statement, including the attached exhibits and schedules, contains additional relevant information about us and the securities. This prospectus does not include all of the information contained in the registration statement. Whenever a reference is made in this prospectus to a contract or other document, the reference is only a summary and you should refer to the exhibits that form a part of the registration statement for a copy of the contract or other document. You can get a copy of the registration statement, at prescribed rates, from the sources listed above. You can also obtain these documents from us, without charge (other than exhibits, unless the exhibits are specifically incorporated by reference), by requesting them in writing or by telephone at the following address:

Investor Relations Manager

Clean Diesel Technologies, Inc.

4567 Telephone Road, Suite 100

Ventura, CA 93003

(805) 639-9460

Internet Website: www.cdti.com

INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The SEC allows us to “incorporate by reference” into this prospectus the information contained in the documents we file with the SEC, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be part of this prospectus, and later information that we file with the SEC will update and supersede this information.

This prospectus incorporates by reference the documents listed below that we have previously filed with the SEC. These documents contain important information about us:

• our Quarterly Report on Form 10-Q for the quarter ending March 31, 2012, which was filed on May 10, 2012;

• our Annual Report on Form 10-K for the year ended December 31, 2011 (including information specifically incorporated by reference into our Form 10-K from our definitive proxy statement for our 2012 annual meeting of stockholders, which was filed on April 23, 2012 and amended on April 23, 2012) filed on March 29, 2012;

• our Current Reports on Form 8-K filed on February 17, 2012, February 28, 2012, March 14, 2012 and May 7, 2012; and

• the description of our Common Stock, $0.01 par value per share, contained in our Registration Statement on Form 8-A filed on September 27, 2007, including any amendments or reports filed for the purpose of updating such description.

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All filings that we may file pursuant to the Exchange Act subsequent to the date hereof and prior to effectiveness of this registration statement shall be deemed to be incorporated in this registration statement and to be a part hereof from the date of filing of such documents or reports. In addition, we incorporate by reference any additional documents that we may file with the SEC under Section 13(a), 13(c), 14 or 15(d) of the Exchange Act (other than those furnished pursuant to Item 2.02 or Item 7.01 of Form 8-K or other information “furnished” to the SEC), from the date of the registration statement of which this prospectus is part until the termination of the offering of the securities. These documents may include annual, quarterly and current reports, as well as proxy statements. Any material that we later file with the SEC will automatically update and replace the information previously filed with the SEC. These documents are available to you without charge. See “Where You Can Find More Information.”

For purposes of this prospectus, any statement contained in a document incorporated or deemed to be incorporated herein by reference shall be deemed to be modified or superseded to the extent that a statement contained herein or in any other subsequently filed document which also is or is deemed to be incorporated herein by reference modifies or supersedes such statement in such document.

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CLEAN DIESEL TECHNOLOGIES INCORPORATED

$50,000,000

Common Stock

Preferred Stock

Warrants

Units

PROSPECTUS

                    , 2012

PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14. Other Expenses Of Issuance And Distribution.

The following table sets forth the fees and expenses, other than underwriting discounts and commissions, payable by the Company in connection with the issuance and distribution of the offered securities. All of the amounts shown are estimates except the SEC registration fee.

SEC registration fee	$	5,730.00
Legal fees and expenses		100,000.00
Accounting fees and expenses		15,000.00
FINRA fee		5,500.00
Printing Fees		1,500.00
Total	$	127,730.00

Item 15. Indemnification of Directors and Officers

Subsection (a) of Section 145 of the Delaware General Corporation Law empowers a corporation to indemnify any person who was or is threatened to be made a party to any threatened, pending or completed action, suit or proceeding, whether civil, criminal, administrative or investigative (other than an action by or in the right of the corporation) by reason of the fact that he or she is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise, against expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by him or her in connection with such action, suit or proceeding if he or she acted in good faith and in a manner he or she reasonably believed to be in or not opposed to the best interests of the corporation, and, with respect to any criminal action or proceeding, had no reasonable cause to believe his or her conduct was unlawful.

Subsection (b) of Section 145 empowers a corporation to indemnify any person who was or is a party or is threatened to be made a party to any threatened, pending or completed action or suit by or in the right of the corporation to procure a judgment in its favor by reason of the fact that he or she is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise, against expenses (including attorneys’ fees) actually and reasonably incurred by him or her in connection with the defense or settlement of such action or suit if he or she acted in good faith and in a manner he or she reasonably believed to be in or not opposed to the best interests of the corporation and except that no indemnification shall be made in respect of any claim, issue or matter as to which such person shall have been adjudged to be liable to the corporation unless and only to the extent that the Court of Chancery or the court in which such action or suit was brought shall determine upon application that, despite the adjudication of liability but in view of all of the circumstances of the case, such person is fairly and reasonably entitled to indemnity for such expenses which the Court of Chancery or such other court shall deem proper.

Section 145 further provides that to the extent a director or officer of a corporation has been successful in the defense of any action, suit or proceeding referred to in subsection (a) and (b) or in the defense of any claim, issue or matter therein, he or she shall be indemnified against expenses (including attorneys’ fees) actually and reasonably incurred by him or her in connection therewith; that the indemnification provided by Section 145 shall not be deemed exclusive of any other rights to which the indemnified party may be entitled; and that the scope of indemnification extends to directors, officers, employees, or agents of a constituent corporation absorbed in a consolidation or merger and persons serving in that capacity at the request of the constituent corporation for another. Section 145 also empowers a corporation to purchase and maintain insurance on behalf of a director or officer of the corporation against any liability asserted against him or her or incurred by him or her in any such capacity or arising out of his or her status as such whether or not the corporation would have the power to indemnify him or her against such liabilities under Section 145.

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Article VIII of Clean Diesel Technologies, Inc.’s Amended Certificate of Incorporation, as amended (“Certificate of Incorporation”), specifies that CDTI shall indemnify its directors, officers, employees and agents to the full extent that such right of indemnity is permitted by law. This provision is deemed to be a contract between CDTI and each director and officer who serves in such capacity at any time while such provision and the relevant provisions of the Delaware General Corporation Law are in effect, and any repeal or modification thereof shall not offset any right to indemnification in respect of action, suit or proceeding theretofore or thereafter brought or threatened based in whole or in part upon any such state of facts. The amendment or repeal of such provision of the Certificate of Incorporation may be effected by the affirmative vote of the holders of a majority in interest of all outstanding capital stock of Clean Diesel entitled to vote, in person or by proxy, at any annual or special meeting in which a quorum is present.

Section 102(b)(7) of the Delaware General Corporation Law enables a corporation in its certificate of incorporation to limit the personal liability of members of its Board of Directors for violation of a director’s fiduciary duty of care. This section does not, however, limit the liability of a director for breaching his or her duty of loyalty, failing to act in good faith, engaging in intentional misconduct or knowingly violating a law, authorizing unlawful payments of dividends or unlawful redemptions or stock purchases as contemplated by Section 174 of Delaware General Corporation Law, or from any transaction in which the director derived an improper personal benefit. This section also will have no effect on claims arising under the federal securities laws.

Clean Diesel’s Certificate of Incorporation limits the liability of its directors as authorized by Section 102(b)(7). To amend such provisions the Company would require the affirmative vote of the holders of a majority of the voting power of all outstanding shares of the capital stock of CDTI.

CDTI has obtained liability insurance for the benefit of its directors and officers which provides coverage for losses of directors and officers for liabilities arising out of claims against such persons acting as directors or officers of Clean Diesel (or any subsidiary thereof) due to any breach of duty, neglect, error, misstatement, misleading statement, omission or act done by such directors and officers, except as prohibited by law.

Item 16. Exhibits

See the Exhibit Index attached to this registration statement that is incorporated herein by reference.

Item 17. Undertakings

(1) 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;

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provided, however, that paragraphs (a)(1)(i), (a)(1)(ii) and (a)(1)(iii) of this section do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the Commission by the registrant 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) 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.

(ii) 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 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;

(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 undersigned registrant undertakes that in a primary offering of securities of the undersigned 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 undersigned 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 undersigned 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 the undersigned registrant or used or referred to by the undersigned registrant;

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

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

(6) The undersigned registrant hereby undertakes that, for purposes of determining any liability under the Securities Act of 1933, each filing of the registrant’s annual report pursuant to section 13(a) or section 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to section 15(d) of the Securities Exchange Act of 1934) that is incorporated by reference in 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 the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Act and will be governed by the final adjudication of such issue.

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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 Ventura, State of California, on the 15th day of May 2012.

CLEAN DIESEL TECHNOLOGIES, INC.
By:	/s/ R. Craig Breese
	R. Craig Breese
	President and Chief Executive Officer

By:	/s/ Nikhil A. Mehta
	Nikhil A. Mehta
	Chief Financial Officer (principal financial officer)

POWER OF ATTORNEY

The undersigned hereby constitutes and appoints R. Craig Breese and Nikhil A. Mehta and each of them, as his true and lawful attorneys-in-fact and agents, jointly and severally, with full power of substitution and re-substitution, for and in his stead, in any and all capacities, to sign on his behalf this Registration Statement on Form S-3 in connection with the registration of Common Stock by the registrant and offering thereof pursuant hereto and to execute any amendments thereto (including post-effective amendments), including a registration statement filed pursuant to Rule 462(b), or certificates that may be required in connection with this Registration Statement, and to file the same, with all exhibits thereto, and all other documents in connection therewith, with the Securities and Exchange Commission and granting unto said attorneys-in-fact and agents, and each of them, jointly and severally, the full power and authority to do and perform each and every act and thing necessary or advisable to all intents and purposes as he might or could do in person, hereby ratifying and confirming all that said attorneys-in-fact and agents, or any of them, jointly or severally, or their or his substitute or substitutes, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed by the following persons in the capacities below.

Signature	Title	Date
/s/ R. Craig Breese R. Craig Breese	President and Chief Executive Officer (principal executive officer)	May 15, 2012
/s/ Nikhil A. Mehta Nikhil Mehta	Chief Financial Officer, (principal financial officer)	May 15, 2012
/s/ David E. Shea David E. Shea	Controller, (principal accounting officer)	May 15, 2012
/s/ Alexander Ellis III Alexander Ellis III	Chairman of the Board	May 15, 2012
/s/ Charles F. Call Charles F. Call	Vice-Chairman	May 15, 2012
/s/ Bernard H. Cherry Bernard H. Cherry	Director	May 15, 2012
/s/ Charles R. Engles, Ph.D. Charles R. Engles, Ph.D.	Director	May 15, 2012
/s/ Derek R. Gray Derek R. Gray	Director	May 15, 2012
/s/ Mungo Park Mungo Park	Director	May 15, 2012

INDEX TO EXHIBITS

Exhibit Number	Description
1.1*	Form of Underwriting Agreement
2.1	Agreement and Plan of Merger, dated as of May 13, 2010, among Clean Diesel Technologies, Inc. (“Clean Diesel”), CDTI Merger Sub, Inc. and Catalytic Solutions , Inc. (incorporated by reference to Annex A to the joint proxy statement/information statement and prospectus included in Clean Diesel’s Registration Statement on Form S-4/A filed on September 23, 2010).
2.2	Letter Agreement dated September 1, 2010 amending the Agreement and Plan of Merger dated as of May 13, 2010 (incorporated by reference to Exhibit 2.2 to the joint proxy statement/information statement and prospectus included in Clean Diesel’s Registration Statement on Form S-4/A filed on September 23, 2010).
2.3	Letter Agreement dated September 14, 2010 amending the Agreement and Plan of Merger dated as of May 13, 2010 (incorporated by reference to Exhibit 2.3 to the joint proxy statement/information statement and prospectus included in Clean Diesel’s Registration Statement on Form S-4/A filed on September 23, 2010).
4.1	Specimen of Certificate for Clean Diesel Common Stock (incorporated by reference to Exhibit 4.1 to Clean Diesel’s Post-Effective Amendment No. 1 to Form S-4 on Form S-3 (No. 333-166865) filed on November 10, 2010).
4.2*	Form of Certificate of Designations for Preferred Stock
4.3*	Form of Warrant Agreement
4.4*	Form of Unit Agreement
5.1	Opinion of DLA Piper LLP (US)
23.1	Consent of KPMG LLP, Independent Registered Public Accounting Firm
23.2	Consent of DLA Piper LLP (US) (included in Exhibit 5.1)
24.1	Power of Attorney (included on signature page hereto)

* To be filed by amendment or as an exhibit to a report pursuant to Section 13(a), 13(c) or 15(d) of the Exchange Act.

EX-5.1

Exhibit 5.1

DLA Piper LLP (us)

2525 East Camelback Road, Suite 1000

Phoenix, Arizona 85016-4232

www.dlapiper.com

T    480.606.5100

F    480.606.5101

May 15, 2012

Board of Directors

Clean Diesel Technologies, Inc.

4567 Telephone Road, Suite 100

Ventura, CA 93003

Ladies and Gentlemen:

We serve as counsel to Clean Diesel Technologies, Inc., a Delaware corporation (the “Company”), and have been requested to render this opinion in connection with the Registration Statement on Form S-3 of the Company (as amended, the “Registration Statement”), filed with the Securities and Exchange Commission (the “SEC”) on May 15, 2012, including the preliminary prospectus included therein at the time the Registration Statement is declared effective (the “Prospectus”), under the Securities Act of 1933, as amended (the “Securities Act”), for registration by the Company of $50,000,000 aggregate initial offering price of Securities (as defined below).

As used herein, the term “Securities” includes (i) shares of common stock, par value $0.01 per share (the “Common Stock”), (ii) shares of preferred stock, par value $0.01 per share (the “Preferred Stock”), (iii) warrants to purchase Preferred Stock (the “Preferred Stock Warrants”) or Common Stock (the “Common Stock Warrants”), as designated by the Company at the time of the offering (collectively, the “Warrants”) and (iv) units comprised of one or more shares of Common Stock, shares of Preferred Stock and/or Warrants, in any combination (the “Units”). The Registration Statement provides that the Securities may be offered separately or together, in separate series, in amounts, at prices and on terms to be set forth in one or more supplements to the Prospectus (each, a “Prospectus Supplement”) or any related free writing prospectus (each, a “Free Writing Prospectus”).

In rendering the opinion expressed herein, we have reviewed originals or copies, certified or otherwise identified to our satisfaction, of the following documents (the “Documents”):

(1) The Registration Statement;

(2) the Restated Certificate of Incorporation, as amended (the “Charter”) and By-laws, as amended (the “By-Laws”), of the Company, certified as true, accurate and complete by an officer of the Company (collectively, the “Organizational Documents”);

(3) a Unanimous Written Consent of the Board of Directors, dated May 15, 2012, certified as true, accurate and complete, and in full force and effect, by an officer of the Company; and

(4) such other certificates, documents and matters as we have deemed necessary and appropriate to render the opinions set forth in this opinion, subject to the limitations, assumptions, and qualifications noted below.

In examining the Documents, we have assumed, without independent investigation, the genuineness of all signatures, the legal capacity of all individuals who have executed any of the aforesaid documents, the authenticity of all documents submitted to us as originals, and the conformity with originals of all documents submitted to us as copies (and the authenticity of the originals of such copies), the absence of other agreements or understandings among the parties that would modify the terms of the proposed transactions or the respective rights or obligations of the parties thereunder and the accuracy and

Board of Directors

Clean Diesel Technologies, Inc.

May 15, 2012

Page Two

completeness of all public records reviewed by us. In making our examination of documents executed by parties other than the Company, we have assumed that such parties had the power and authority (corporate, trust, partnership or other) to enter into and perform all obligations thereunder, and we have also assumed the due authorization by all requisite action (corporate, trust, partnership or other) and the valid execution and delivery by such parties of such documents and the validity, binding effect and enforceability thereof with respect to such parties. As to any facts material to this opinion, we have relied solely upon the Certificate.

We further assume that:

(1) The issuance, sale, amount and terms of the Securities to be offered from time to time by the Company will be authorized and determined by proper action of the Board of Directors (or where permitted, a committee of the Board of Directors) of the Company (each, a “Board Action”) in accordance with the Company’s Organizational Documents and applicable law, in each case so as not to result in a default under or breach of any instrument, document or agreement binding upon the Company and so as to comply with any requirement or restriction imposed by any court or governmental or regulatory body having jurisdiction over the Company.

(2) The Company will not issue any Securities in excess of the number or amount authorized by a Board Action.

(3) Prior to the issuance of any shares of Preferred Stock (including Preferred Stock that is the subject of Preferred Stock Warrants), or Common Stock (including Common Stock that is the subject of Common Stock Warrants or convertible Preferred Stock), there will exist, under the Company’s Charter, the requisite number of authorized but unissued shares of Preferred Stock (and securities of any class into which any of the Preferred Stock may be convertible), or Common Stock, as the case may be, and that all actions necessary to the creation of any such Preferred Stock (and securities of any class into which any Preferred Stock may be convertible), whether by certificates of designation or by classification or reclassification of existing capital stock and the filing of amendments to the Company’s Charter, will have been taken.

(4) For shares of Preferred Stock or Common Stock represented by certificates (“Certificates”), appropriate Certificates representing shares of Preferred Stock or Common Stock will be executed and delivered upon issuance and sale of any such shares of Preferred Stock or Common Stock, as the case may be, and will comply with the Company’s Charter and By-Laws and applicable law. For shares of Preferred Stock or Common Stock not represented by certificates, the applicable Board Action shall have been taken and, upon request of a stockholder of the Company, appropriate written statements (“Written Statements”) will be prepared and delivered to such stockholder upon issuance and sale of any such shares of Preferred Stock or Common Stock, as the case may be, and will comply with the Company’s Charter and By-Laws and applicable law.

(5) Any Warrants will be issued under a valid and legally binding warrant agreement (a “Warrant Agreement”) that conforms to the description thereof set forth in the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus and will comply with the Company’s Charter and By-Laws and applicable law.

Board of Directors

Clean Desel Technologies, Inc.

May 15, 2012

Page Three

(6) Any Units will be issued under a valid and legally binding unit agreement (a “Unit Agreement”) that conforms to the description thereof set forth in the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus and will comply with the Company’s Charter and By-Laws and applicable law.

(7) The underwriting or other agreements for offerings of the Securities (each, an “Underwriting Agreement,” and collectively, the “Underwriting Agreements”) will be valid and legally binding contracts that conform to the description thereof set forth in the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus.

(8) The Registration Statement will have been declared effective and will remain effective under the Securities Act.

Based upon the foregoing, and subject to the assumptions, limitations and qualifications stated herein, it is our opinion that:

(a) Upon an issuance of Certificates or Written Statements, if any, as the case may be, representing shares of Common Stock, against payment therefor in accordance with the terms and provisions of the applicable Board Action, the terms of the Charter, By-Laws and applicable law, the Registration Statement, the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, or upon issuance and delivery of Certificates or Written Statements, if any, for shares of Common Stock pursuant to the exercise of one or more Common Stock Warrants or the conversion of one or more series of Preferred Stock convertible into Common Stock, such shares of Common Stock will be duly authorized, validly issued, fully paid and non-assessable.

(b) Upon an issuance of Certificates or Written Statements, if any, as the case may be, representing shares of Preferred Stock, against payment therefor in accordance with the terms and provisions of the applicable Board Action, the terms of the Charter, By-Laws and applicable law, the Registration Statement, the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, or upon issuance and delivery of Certificates or Written Statements, if any, for shares of Preferred Stock pursuant to the exercise of one or more Preferred Stock Warrants, such shares of Preferred Stock will be duly authorized, validly issued, fully paid and non-assessable.

(c) Upon execution, issuance, and delivery of the Warrants against payment therefor in accordance with the terms and provisions of the applicable Board Action, the Charter, the By-Laws and applicable law, the Warrant Agreement, the Registration Statement, the Prospectus, the applicable Prospectus Supplement and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, the Warrants will be duly authorized and constitute valid and legally binding obligations of the Company.

Board of Directors

Clean Diesel Technologies, Inc.

May 15, 2012

Page Four

(d) Upon execution, issuance, and delivery of the Units against payment therefor in accordance with the terms and provisions of the applicable Board Action, the Charter, the By-Laws and applicable law, the Unit Agreement, the Registration Statement, the Prospectus, the applicable Prospectus Supplement, and any related Free Writing Prospectus, and, if applicable, an Underwriting Agreement, and assuming that the underlying securities that are components of such Units have been duly and properly authorized for such issuance and constitute valid and binding obligations enforceable against the issuer thereof in accordance with their terms, the Units will be duly authorized and constitute valid and legally binding obligations of the Company.

In addition to the qualifications assumptions and limitations set forth above, the foregoing opinion is further qualified as follows:

(1) We do not express any opinion herein concerning any law other than the Delaware General Corporation Law and the substantive federal securities laws of the United States of America as in effect on the date hereof. We express no opinion as to the laws of any other state or jurisdiction of the United States or of any foreign jurisdiction. We have made no inquiry into the laws and regulations or as to laws relating to choice of law or conflicts of law principles. The opinion expressed herein is subject to the effect of judicial decisions which may permit the introduction of parol evidence to modify the terms or the interpretation of agreements.

(2) The opinion stated herein relating to the validity and binding nature of obligations of the Company is subject to (i) the effect of any applicable bankruptcy, insolvency (including, without limitation, all laws relating to fraudulent transfers), reorganization, fraudulent conveyance, moratorium or similar laws affecting creditors’ rights generally and (ii) the effect of general principles of equity (regardless of whether considered in a proceeding in equity or at law).

We hereby consent to the filing of this opinion with the SEC as Exhibit 5.1 to the Registration Statement and to the reference to our firm under the heading “Legal Matters” in the Registration Statement. In giving our consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the SEC thereunder.

Very truly yours,

DLA Piper LLP (US)

EX-23.1

Exhibit 23.1

Consent of Independent Registered Public Accounting Firm

The Board of Directors

Clean Diesel Technologies, Inc.:

We consent to the use of our report dated March 29, 2012, with respect to the consolidated balance sheets of Clean Diesel Technologies, Inc. as of December 31, 2011 and 2010, and the related consolidated statements of operations, stockholders’ equity and comprehensive loss, and cash flows for the years then ended, incorporated herein by reference and to the reference to our firm under the heading “Experts” in the prospectus.

/s/ KPMG LLP

Los Angeles, California

May 15, 2012