424B5

We are offering 2,030,000 shares of our common stock and warrants to purchase 812,000 shares of our common stock in this offering. Each share of common stock is being sold together with 0.4 of one warrant to purchase one share of common stock at an exercise price of $4.20 per share of common stock. The warrants can be exercised during the period commencing after the date occurring six months from the date of issuance and ending on five and a half years from the date of issuance. The shares of common stock and warrants will be issued separately. There is no established public trading market for the warrants, and we do not expect a market to develop. In addition, we do not intend to apply for listing of the warrants on any national exchanges or other nationally recognized trading system.

Our common stock is listed on the NASDAQ Capital Market under the symbol “CDTI.” On March 31, 2014, the last reported sale price of our shares of common stock was $3.80 per share.

Based on the reported sale price of $6.33 of our common stock on the NASDAQ Capital Market on March 18, 2014, the aggregate market value of our public float, calculated according to General Instruction I.B.6 of Form S-3, is approximately $60.7 million. Under the registration statement to which this prospectus supplement forms a part, we may not sell our securities in a primary offering with a value exceeding one-third of our public float in any 12-month period (unless our public float rises to $75.0 million or more). We have offered securities for an aggregate market value of $5,687,300 during the 12 calendar months preceding and including the date of this prospectus supplement pursuant to General Instruction I.B.6, and, accordingly, may sell up to $14,546,661 in stock and warrants hereunder.

We have engaged Roth Capital Partners, LLC and Craig-Hallum Capital Group LLC as our exclusive placement agents in connection with this offering. The placement agents have no obligation to buy any of the securities from us or to arrange for the purchase or sale of any specific number or dollar amount of securities. See “Plan of Distribution” beginning on page S-21 of this prospectus supplement for more information regarding these arrangements.

Investing in our securities involves a high degree of risk. We refer you to the section entitled “Risk Factors” on page S-8 of this prospectus supplement and on page 3 of the accompanying prospectus 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 passed upon the adequacy or accuracy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

This document is part of a registration statement that we filed with the Securities and Exchange Commission (“SEC”) using a “shelf” registration process and consists of two parts. The first part is this prospectus supplement, which describes the terms of the securities being offered by us and also adds to and updates information contained in the accompanying prospectus as well as the documents incorporated by reference into this prospectus supplement and the accompanying prospectus. The second part, the accompanying prospectus, gives more general information about securities we may offer from time to time. To the extent any inconsistency or conflict exists between the information included in this prospectus supplement and the information included in the accompanying prospectus, the information included or incorporated by reference in this prospectus supplement updates and supersedes the information in the accompanying prospectus. Generally, when we refer only to the “prospectus,” we are referring to both parts combined. This prospectus supplement incorporates by reference important business and financial information about us that is not included in or delivered with this prospectus supplement.

It is important for you to read and consider all information contained in this prospectus supplement and the accompanying prospectus in making your investment decision. You should also read and consider the information contained in the documents identified under the headings “Incorporation of Certain Documents by Reference” and “Where You Can Find More Information.”

You should rely only on the information provided in this prospectus supplement and the accompanying prospectus, including the information incorporated by reference. We have not authorized anyone to provide you with different information. You should not assume that the information contained or incorporated by reference in this prospectus supplement or the accompanying prospectus is accurate as of any date other than as of the date of this prospectus supplement or the accompanying prospectus, as the case may be, or in the case of the documents incorporated by reference, the date of such documents regardless of the time of delivery of this prospectus supplement and the accompanying prospectus or any sale of our securities. We are not making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted.

Unless otherwise indicated or the context otherwise requires, 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.

PROSPECTUS SUPPLEMENT SUMMARY

This summary highlights certain information contained elsewhere in this prospectus supplement or incorporated by reference herein. This summary does not contain all of the information that you should consider before investing in our securities. You should read the entire prospectus supplement and the accompanying prospectus carefully, including the risks related to our business and investing in our securities discussed under “Risk Factors” beginning on page S-9 and the other information and documents incorporated by reference into this prospectus supplement, including our consolidated financial statements and related notes thereto.

Overview

We are a leading technology-focused, global manufacturer and distributor of light duty vehicle catalysts and heavy duty diesel emissions control systems and products to major automakers, integrators and retrofitters. We have more than 13 years history of supplying catalysts to light duty vehicle original equipment manufacturers, or OEMs, and over 30 years of experience in the heavy duty diesel systems market.

We organize our operations in two primary divisions: our Catalyst division and our Heavy Duty Diesel Systems division.

Catalyst: Our Catalyst division develops and produces catalysts to reduce emissions from gasoline, diesel and natural gas combustion engines. Most catalytic systems require significant amounts of costly platinum group metals, or PGMs, to operate efficiently. Using our proprietary mixed-phase catalyst, or MPC^®, technology, we have developed a family of unique high-performance catalysts, featuring inexpensive base-metals with low or even no PGM content. Our technical and manufacturing capabilities have been established to meet auto makers’ most stringent requirements. Since 2001, we have supplied over eleven million parts to light duty vehicle customers. Our Catalyst division also provides catalyst products for our Heavy Duty Diesel Systems division. We expect growth in this division to be driven by increased sales to existing customers, including Honda, and to new OEMs seeking high performance solutions that alleviate the cost of high PGM found in standard offerings. In addition, we will continue to drive sales internally to 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 non-retrofit OEM and aftermarket markets through its distribution/dealer network and direct sales. Our Purifilter^®, Purifier™, Combifilter^®, Cattrap^® and Actifilter™ products, along with our catalyst 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. We also provide Platinum Plus^® fuel-borne catalyst technology, ARIS^® airless return flow system technology and exhaust gas recirculation with selective catalyst reduction technologies. We believe we offer one of the industry’s most comprehensive portfolios of emission control systems for use in engine retrofit programs that have been evaluated and verified as compliant with applicable regulations by the U.S. Environmental Protection Agency and the California Air Resources Board, as well as by regulators in several European countries. Sales of emission control systems by our Heavy Duty Diesel Systems division are driven by the regulation of diesel emissions, particularly in the State of California.

We are headquartered, in Ventura, California and have operations in the United States, Canada, France, Japan and Sweden as well as an Asian investment. Our proprietary catalyst products are manufactured at our facility in Oxnard, California and our heavy duty diesel systems and products are manufactured at our facilities in Reno, Nevada; Thornhill, Canada; and Malmö, Sweden.

Recent Developments

Technology Strategy

Our board of directors and management team conducted a strategic review of our business and determined to pursue aggressive development of our unique materials science platform, which we view as the most likely path to enhance growth and improve shareholder value over the long-term. The new strategy is intended to build on recent initiatives and announcements, including an increased focus on our proprietary low- and zero-platinum technologies. We intend to pursue additional licensing and partnership arrangements to accelerate the commercialization of our patented and proprietary materials technology and reduce our need for additional, significant capital expenditures to expand our manufacturing platform. Based on our strategic review, we have defined our strategic priorities as follows:

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Explore strategic options to maximize the value of our manufacturing assets and business;

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Focus our research and development efforts on new technology development, patent protection and commercialization of new advanced low- PGM and zero PGM (ZPGM) materials and robust manufacturing process technology;

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Aggressively build our patent portfolio to maintain and protect our technology leadership position;

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Develop and qualify emission catalysts for a variety of applications in multiple segments of the emission control market;

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Seek customers or partners for core emission control technologies via development partnerships, licensing, joint venture or manufacturing agreements and pursue short-term catalyst sales opportunities; and

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Pursue new end markets, including fuel cells, petro-chemicals and thermo-electrics.

To date, we have not developed any licensing arrangements or new OEM customers, and we have terminated our joint venture with Pirelli. We have not made any final decisions regarding our manufacturing operations. There can be no assurance that any of our strategic priorities will be implemented or that, if implemented, they will be successful. We continually review our strategies and could change them at any time. See “Risk Factors.”

Primary Customer

We remain dependent on our primary customer, Honda, for which we provide catalyst solutions. Our business with Honda has grown steadily in the last few years as we have expanded the sale of our catalyst solution from two passenger vehicle models in 2011 to six models in 2013. On March 11, 2014, we announced that we have begun production of catalysts featuring our high-performance Mixed Phase Catalyst (MPC^®) technology for Honda’s 2015 Acura TLX model of which we have been named Honda’s catalyst supplier. Shipments are expected to commence in this first half of 2014.

In conjunction with our longstanding relationship with Honda, we entered into a joint research agreement with the motorcycle division of Honda regarding the development of ZPGM catalysts for motorcycles. The agreement was signed in 2010, extended in 2012 and expires on March 31, 2014, although confidentiality provisions survive thereafter. The agreement provides that technology within the scope of the agreement developed solely by one party is owned by that party, and that technology within the scope of the agreement that is jointly developed by both parties is jointly owned. Beginning in April 2014, the parties plan to assess what technology, if any, developed during the term of the agreement is jointly owned. To the extent that Honda is a joint owner of critical technology developed under the agreement, Honda (including its automotive division) might not be required to pay us a license or royalty fee for use of the jointly owned technology; Honda may be able to manufacture its own catalysts based on the jointly owned technology; and Honda may be able to license

the jointly owned technology to others without our consent. In addition, under the terms of the agreement, we may not be able to license jointly owned technology to others without Honda’s consent. Our inability to license jointly owned technology to others could adversely affect our technology licensing strategy. We do not have long-term supply agreements with Honda and accordingly Honda could terminate its relationship with us at any time for any reason.

Asian Investment

In February 2008, we established a joint venture in Japan called TC Catalyst, Inc., or TCC, with Tanaka Holdings Kabushiki Kaisha, a Japanese company, which, together with its subsidiary Tanaka Kikinzoku Kogyo Kabushiki Kaisha, is referred to herein as TKK. Initially, we and TKK each owned 50% of TCC, but since formation we have sold most of our stake in the venture to TKK and now own 5%. In connection with these transactions, we also sold to TKK certain proprietary technology for sale, licensing or use in various countries in Asia, which we refer to herein as the Territory. In general, the technology covers our catalyst formulations (including platinum and zero platinum) developed for heavy duty commercial vehicles and other applications through 2013, and for non-commercial light vehicles through 2012. In addition, TKK has a right to cause us to license heavy duty commercial technology to TKK or TCC in exchange for a royalty if TKK or TCC desire to sell related products or services outside the Territory to subsidiaries of OEM customers located within the Territory. We have also agreed not to compete in the Territory with TKK or TC Catalyst in connection with heavy duty commercial vehicles and applications and light duty vehicles.

We currently sell our catalyst products primarily in the U.S., and do not plan on selling our products in the Territory in the near term due to the additional burdens and resources involved in establishing a local manufacturing presence and other factors. However, if circumstances change, and selling our current products in the Territory in the near term becomes desirable, we currently would be prohibited from doing so, which could adversely affect our ability to expand unless we obtain a license and/or waiver from TKK.

In addition, the Company recently discovered that an exception allowing it to continue to supply catalysts in Japan to its largest customer, Honda, had been omitted in an amendment to the original transaction documents with TKK. The Company has shipped $8.5 million of catalysts covered by the agreements since such amendment. The Company is in discussions with TKK regarding correcting this omission, but, in order to do so, may have to pay TKK a royalty on past and future sales of catalysts covered by the agreement. If the Company is unable to reach an agreement with TKK, the Company would likely have to suspend shipments of catalysts to Japan to its largest customer, which could adversely affect its relationship with its largest customer, as well as its business and financial performance. In addition, TKK could sue for breach of the agreements, to enjoin future shipments into Japan if we fail to discontinue them voluntarily, and/or for any other available remedies, any of which (including the mere cost to defend such a lawsuit) could have a material adverse effect on our business and financial performance.

Going Concern

The report of our independent auditors on our financial statements for the period ended December 31, 2013 included an explanatory paragraph indicating that there is substantial doubt about our ability to continue as a going concern. Our auditor’s doubts are based on our inability to establish an ongoing source of revenue sufficient to cover our operating costs and recurring losses from operations. Although there can be no assurance that our request will be granted, we will seek to have the going concern qualification withdrawn upon the completion of this offering. For more information, see “Risk Factors.”

The Offering

Issuer

Clean Diesel Technologies, Inc.

Common stock offered

2,030,000 shares of our common stock, plus 812,000 shares of our common stock underlying the warrants offered pursuant to this offering.

Warrants offered

Each share of common stock is being sold together with 0.4 of one warrant to purchase one share of common stock at an exercise price of $4.20 per share. The warrants can be exercised during the period commencing after the date occurring six months from the date of issuance and ending five and a half years from the date of issuance. This prospectus supplement also relates to the offering of the shares of common stock issuable upon exercise of the warrants. There is no established public trading market for the warrants, and we do not expect a market to develop. In addition, we do not intend to apply for listing of the warrants on any national securities exchange or other nationally recognized trading system. See “Description of Securities” on page S-19.

Common stock to be outstanding immediately following this offering (1)

12,180,575 shares, or 12,992,575 shares of our common stock if the warrants offered pursuant to this offering are exercised in full.

Use of proceeds

We estimate that the net proceeds from the sale of our securities in this offering will be approximately $6.1 million, after deducting placement agent fees and expense reimbursements and our estimated expenses related to this offering.

We intend to use our net proceeds from this offering for general corporate purposes, which may include working capital, general and administrative expenses, capital expenditures and implementation of our strategic priorities. See “Use of Proceeds” on page S-16.

Risk factors

This investment involves a high degree of risk. See “Risk Factors” for a discussion of factors you should consider carefully before making an investment decision.

NASDAQ Capital Market ticker symbol

CDTI.

(1)	The number of shares outstanding immediately following the offering is based on 10,150,575 shares of common stock outstanding as of March 25, 2014 and excludes as of such date unless otherwise noted:

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635,208 shares of common stock subject to outstanding options with a weighted average exercise price of $7.71 per share;

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381,987 shares subject to unvested restricted stock units with a weighted average grant date fair value of $2.98 per share;

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59,589 shares subject to vested and unissued restricted stock units with a weighted average granted date fair value of $2.49 per share;

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339,535 shares of common stock issuable upon exercise of outstanding warrants (which does not include the warrants offered hereby) at an average weighted exercise price of $2.70 per share;

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250,000 shares of common stock potentially issuable upon the conversion of convertible notes;

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217,832 shares reserved for future issuance under our equity incentive plan; and

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the shares of common stock issuable upon the exercise of the warrants offered hereby.

Summary Consolidated Financial Data

The following table sets forth our summary consolidated financial data for the periods indicated. We have derived our summary consolidated statement of operations data for the years ended December 31, 2012 and 2013 and summary consolidated balance sheet data as of December 31, 2013 from our audited consolidated financial statements included in our annual report on Form 10-K for the year ended December 31, 2013 and incorporated by reference in this prospectus supplement. You should read this information together with “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and the financial statements and related notes incorporated by reference into this prospectus supplement. Our historical results are not necessarily indicative of the results to be expected in any future period.

Consolidated Statement of Operations Data (in thousands, except per share data):


	Year Ended December 31,
	2012			2013
Revenue	$	60,537		$	55,284
Cost of revenue		45,816			39,880
Net loss from continuing operations (1)		(9,694	)		(6,780	)
Net loss per share:
Net loss from continuing operations per share		(1.34	)		(0.82	)
Net loss per share		(1.34	)		(0.86	)

Consolidated Balance Sheet Data (in thousands):


	As of December 31, 2013
Cash	$	3,909
Working capital		2,126
Indebtedness		9,807
Stockholders’ equity		5,446

(1)

On October 1, 2009, the Company sold all significant assets of Applied Utility Systems, Inc., a provider of solutions for the clean and efficient utilization of fossil fuels, for cash and contingent consideration, which has not yet been earned and will not be paid. The (loss) income, net of tax, of the remaining assets is presented as discontinued operations.

You should carefully consider the risk factors set forth below, under the caption “Risk Factors” in the accompanying prospectus and under the caption “Risk Factors” in our Annual Report on Form 10-K for the fiscal year ended December 31, 2013, which is incorporated by reference in this prospectus supplement and the accompanying prospectus. 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 supplement and the accompanying prospectus. The risks and uncertainties we describe are not the only ones facing us. 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 common stock 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.

We cannot assure you that we will be successful in realigning our strategic path.

Our board of directors and management team conducted a strategic review of our business and determined to pursue aggressive development of our unique materials science platform. The new strategy contemplated an increased focus on patenting our proprietary advanced low-PGM and zero PGM, or ZPGM, catalysts. We determined to pursue additional licensing and partnership arrangements to accelerate the commercialization of our patented and proprietary materials technology and reduce our need for additional, significant capital expenditures to expand our manufacturing platform.

We cannot assure you that these efforts will be successful and, if they are, that they will have the intended effect of increasing profitability. We may not be able to implement these strategies for a number of reasons, including, but not limited to:

In this regard, we have not developed any licensing arrangements or new OEM customers to date, and, as a result of slower than anticipated progress in achieving sales objectives initially established for our joint venture with Pirelli, we and Pirelli agreed to voluntarily dissolve our joint venture. The joint venture ceased operations on November 30, 2013 and commenced liquidation on December 9, 2013. We expect that dissolution will be finalized in the first half of 2014. We have also not made any final decisions regarding our manufacturing operations. Moreover, we believe some of these initiatives to be relatively uncommon in our industry, and, as a result, are unfamiliar to us. The success of our business assumes we will be able to execute these strategies and increase our profitability as a result. This assumption is unproven, and, if incorrect, we may be unable to generate sufficient revenues to sustain our business, implement or business strategies, or to obtain profitability.

Furthermore, in attempting to execute these strategies, we may harm our relationships with customers, suppliers, employees or other third parties, any of which could be significant. The process of exploring, financing, and realigning our strategic path may also be disruptive to our business. While we believe the pursuit of these strategies will have a positive effect on our profitability in the long-term, there is no assurance that this will be the case. If we are not successful in our efforts to carry out these strategies, our business, financial condition, and results of operation may be adversely affected.

Historically, we have been dependent on a few major customers, particularly Honda, for a significant portion of our revenue and our revenue could decline if we are unable to maintain those relationships, if customers reduce their orders for their products, or if we are unable to secure new customers. In addition, we have an agreement with Honda that could limit our rights to commercialize certain technology which could adversely affect our technology licensing strategy.

Historically, we have derived a significant portion of our revenue from a limited number of customers. For the years ended December 31, 2013 and 2012, sales to Honda, our largest customer, accounted for approximately 39% and 30%, respectively, of our revenue. While we continually seek to broaden our customer base, it is likely that for the foreseeable future we will remain dependent on Honda to supply a substantial portion of our revenue. Manufacturers typically seek to have two or more sources of critical components; however, there can be no assurance that manufacturers for which we are a shared supplier will not sole source the products we supply. Once our product is designed into a vehicle model, we generally supply our component for the life of that model. There can be no assurance, however, that our customers will retain us for a full model term. In this regard, relationships with our customers are based on purchase orders rather than long-term formal supply agreements and customers can discontinue or materially reduce orders without warning or penalty. In addition, while new models tend to remain relatively stable for a few years, there can be no assurance that manufacturers will not change models more rapidly, or change the performance requirements of components used in those models, and use other suppliers for these new or revised models. Demand for our products is tied directly to demand for vehicles. Accordingly, factors that affect the truck and automobile markets have a direct effect on our business, including factors outside of our control, such as vehicle sales slowdowns due to economic concerns, or as a result of natural disasters, including earthquakes and/or tsunamis. The loss of one or more of our significant customers, or reduced demand from one or more of our significant customers, particularly Honda, would result in an adverse effect on our revenue, and could affect our ability to become profitable or continue our business operations.

In conjunction with our longstanding relationship with Honda, we entered into a joint research agreement with the motorcycle division of Honda regarding the development of zero precious metals catalysts for motorcycles. The agreement was signed in 2010, extended in 2012 and expires on March 31, 2014, although confidentiality provisions survive thereafter. The agreement provides that technology within the scope of the agreement developed solely by one party is owned by that party, and that technology within the scope of the agreement that is jointly developed by both parties is jointly owned. Beginning in April 2014, the parties plan to assess what technology, if any, developed during the term of the agreement is jointly owned. To the extent that Honda is a joint owner of critical technology developed under the agreement, Honda (including its automotive division) might not be required to pay us a license or royalty fee for use of the jointly owned technology; Honda may be able to manufacture its own catalysts based on the jointly owned technology; and Honda may be able to license the jointly owned technology to others without our consent. In addition, under the terms of the agreement, we may not be able to license jointly owned technology to others without Honda’s consent. Our inability to license jointly owned technology to others could adversely affect our technology licensing strategy. Further, as noted above, we do not have long-term supply agreements with Honda and accordingly Honda could terminate its relationship with us at any time for any reason.

We are not able to sell our current products in certain countries in Asia since such products are based on technology which we sold to a third party.

In February 2008, we established a joint venture in Japan called TC Catalyst, Inc., or TCC, with Tanaka Holdings Kabushiki Kaisha, a Japanese company, which, together with its subsidiary Tanaka Kikinzoku Kogyo Kabushiki Kaisha, is referred to herein as TKK. Initially, we and TKK each owned 50% of TCC, but since formation we have sold most of our stake in the venture to TKK and now own 5%. In connection with these transactions, we also sold to TKK certain proprietary technology for sale, licensing or use in various countries in Asia (the “Territory”). In general, the technology covers our catalyst formulations (including platinum and zero platinum) developed for heavy duty commercial vehicles and other applications through 2013, and for non-commercial light vehicles through 2012. In addition, TKK has a right to cause us to license heavy duty commercial technology to TKK or TCC in exchange for a royalty if TKK or TCC desire to sell related products or services outside the Territory to subsidiaries of OEM customers located within the Territory. We have also agreed not to compete in the Territory with TKK or TC Catalyst in connection with heavy duty commercial vehicles and applications and light duty vehicles.

We depend on intellectual property and the failure to protect our intellectual property could adversely affect our future growth and success.

We rely on patent, trademark and copyright law, trade secret protection, and confidentiality and other agreements with employees, customers, partners and others to protect our intellectual property. In addition, some of our intellectual property is not protected by any patent or patent application. The lack of patent and trademark protection may be intentional as we may lack sufficient resources to protect our intellectual property in every applicable jurisdiction. As a result, it may be possible for third parties to obtain and use our intellectual property without the need to obtain our authorization.

We do not know whether any patents will be issued from our pending or future patent applications or whether the scope of any issued patents is or will be sufficiently broad to protect our technologies. Moreover, patent applications and issued patents may be challenged or invalidated. We could incur substantial costs in prosecuting or defending patent infringement suits. In addition, the laws of some foreign countries may not protect or enforce intellectual property rights to the same extent as do the laws of the United States.

The patents protecting our proprietary technologies expire after a period of time. Currently, our patents have expiration dates ranging from 2014 through 2030. Although we have attempted to incorporate technology from our core patents into specific patented product applications, product designs and packaging, there can be no assurance that this building block approach will be successful in protecting our proprietary technology and products. If we are not successful in protecting our proprietary technology, it could have a material adverse effect on our business, financial condition and results of operations. The patents protecting our proprietary technologies expire after a period of time. Currently, our patents have expiration dates ranging from 2014 through 2030. Although we have attempted to incorporate technology from our core patents into specific patented product applications, product designs and packaging, there can be no assurance that this building block approach will be successful in protecting our proprietary technology and products. If we are not successful in protecting our proprietary technology, it could have a material adverse effect on our business, financial condition and results of operations. In recent periods, questions have arisen regarding our exclusive ownership and control of certain technologies, including by our principal customer, Honda, and a former employee, who claims ownership in a patent relating to ZPGM. In addition, we have sold technology for exclusive use in Asia to another party. See “—Historically, we have been dependent on a few major customers, particularly Honda…” and “—We may not be able to sell our current products in certain countries in Asia since such products are based on technology which we sold to a third party” above. Past or future weaknesses in control of our intellectual property could render unachievable our current strategies, require that we change our strategies which could prove unsuccessful, result in litigation over ownership issues including the costs thereof and potential adverse findings, require that we pay to license back technology that we developed or co-developed, or otherwise material adversely affect us, our business and our financial performance.

As part of our confidentiality procedures, we generally have entered into nondisclosure agreements with employees, consultants and corporate partners. We also have attempted to control access to and distribution of our technologies, documentation and other proprietary information. We plan to continue these procedures. Despite these procedures, third parties could copy or otherwise obtain and make unauthorized use of our technologies or independently develop similar technologies. The steps that we have taken and that may occur in the future might not prevent misappropriation of our solutions or technologies, particularly in foreign countries where laws or law enforcement practices may not protect the proprietary rights as fully as in the United States.

There can be no assurance that we will be successful in enforcing our proprietary rights. For example, from time to time we have become aware of competing technologies employed by third parties who might be covered by one or more of our patents. In such situations, we may seek to grant licenses to such third parties or seek to stop the infringement, including through the threat of legal action. There is no assurance that we would be successful in negotiating a license agreement on favorable terms, if at all, or able to stop the infringement. Any infringement upon our intellectual property rights could have an adverse effect on our ability to develop and sell commercially competitive systems and components.

Our independent registered public accounting firm has expressed substantial doubt about our ability to continue as a going concern.

The report of our independent auditors on our financial statements for the period ended December 31, 2013, included an explanatory paragraph indicating that there is substantial doubt about our ability to continue as a going concern. Our auditors’ doubts are based on our inability to establish an ongoing source of revenue sufficient to cover our operating costs and recurring losses from operations. Our ability to continue as a going concern will be determined by our ability to generate sufficient cash flow to sustain our operations and/or raise additional capital in the form of debt or equity financing. Our financial statements do not include any adjustments relating to the recoverability and classification of recorded asset amounts or amounts and classification of liabilities that might be necessary should we be unable to continue as a going concern, and shareholders could lose their entire investment in the Company.

There has been and may continue to be significant volatility in the volume and price of our common stock on the NASDAQ Capital Market.

The market price of our common stock has been and may continue to be highly volatile. Factors, including announcements of new business initiatives, technological innovations by us or other companies, regulatory matters, new or existing products or procedures, concerns about our financial position, operations results, litigation, government regulation, developments or disputes relating to agreements, patents or proprietary rights, may have a significant impact on the market volume and price of our stock. Unusual trading volume in our shares occurs from time to time.

We have not paid and do not intend to pay dividends on shares of our common stock. Investors in this offering may never obtain a return on their investment.

We have not paid dividends on our common stock since inception, and do not intend to pay any dividends to our stockholders in the foreseeable future. We intend to reinvest earnings, if any, in the development and expansion of our business. Accordingly, you will need to rely on sales of your common stock after price appreciation, which may never occur, in order to realize a return on your investment.

Since we have broad discretion in how we use the proceeds from this offering, we may use the proceeds in ways with which you disagree.

Our management will have significant flexibility in applying the net proceeds of this offering. You will be relying on the judgment of our management with regard to the use of these net proceeds, and you will not have the opportunity, as part of your investment decision, to assess whether the proceeds are being used appropriately. It is possible that the net proceeds will be invested in a way that does not yield a favorable, or any, return for us. The failure of our management to use such funds effectively could have a material adverse effect on our business, financial condition, prospects, financial condition, operating results and cash flow.

You will experience immediate dilution in the book value per share of the common stock you purchase.

Because the public offering price per share of common stock is expected to be substantially higher than the book value per share of our common stock, you will suffer substantial dilution in the net tangible book value of the common stock you purchase in this offering. Based on the public offering price of $3.40 per share and corresponding warrant, if you purchase shares of common stock and warrants in this offering, you will suffer immediate and substantial dilution of approximately $3.21 per share in the net tangible book value of the common stock you acquire. In the event that you exercise your warrants, you will experience additional dilution to the extent that the exercise price of those warrants is higher than the book value per share of our common stock.

We are limited, and may be further limited by this offering, from taking advantage of our significant U.S. federal tax loss carryforwards and tax credits.

On October 15, 2010, we completed a business combination with CSI when our wholly-owned subsidiary merged with and into CSI, which we refer to herein as the Merger. In connection with the Merger, we performed a study to evaluate the status of net operating loss carryforwards as a result of the Merger. Because the Merger caused an “ownership change” (as defined for U.S. federal income tax purposes) as of the date of the Merger, our ability to use our net operating losses and credits in future tax years has been significantly limited. In addition, due to the “ownership change,” our federal research and development credits have also been limited and, consequently, we do not anticipate being able to use any of these credits that existed as of the date of the Merger in future tax years. In addition, this offering, alone or in combination with other previous offerings of our equity securities, may further limit our ability to use our net operating losses and credits in future tax years. Our limited ability to use these net operating losses and tax credits could have an adverse effect on our results of operations.

Provisions of Delaware law and provisions of our restated certificate of incorporation, as amended, and by-laws could make it more difficult for a third party to acquire control of us or have the effect of discouraging a third party from attempting to acquire control of us. Additionally, our restated certificate of incorporation, as amended, authorizes our board of directors to issue series of preferred stock and such preferred stock could be issued as a defensive measure in response to a takeover proposal. These provisions could make it more difficult for a third party to acquire us even if an acquisition might be in the best interest of our stockholders.

There is no public market for the warrants to purchase common stock in this offering.

There is no established public trading market for the warrants being offered in this offering, and we do not expect a market to develop. In addition, we do not intend to apply to list the warrants on any securities exchange. Without an active market, the liquidity of the warrants will be limited.

Holders of our warrants will have no rights as a common stockholder until such holders exercise their warrants and acquire our common stock.

Until holders of the warrants acquire shares of our common stock upon exercise of the warrants, holders of the warrants will have no rights with respect to the shares of our common stock underlying such warrants. Upon exercise of the warrants, the holders thereof will be entitled to exercise the rights of a common stockholder only as to matters for which the record date occurs after the exercise date.

This prospectus supplement and the information incorporated by reference in this prospectus supplement 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 supplement 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 supplement. 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.

We estimate that the net proceeds from the sale of the shares of our common stock and warrants to purchase shares of common stock in this offering will be approximately $6.1 million, after deducting the placement agents’ fees and estimated expenses related to this offering. In addition, we may receive proceeds of up to $3.4 million if the warrants issued in this offering are exercised for cash. We intend to use the net proceeds from this offering for general corporate purposes, which may include working capital, general and administrative expenses, capital expenditures and implementation of our strategic priorities. 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. Pending the application of the net proceeds, we may invest the proceeds in short-term, interest bearing, investment-grade marketable securities or money market obligations.

As of the date of this prospectus supplement, we cannot specify with certainty all of the particular uses of the proceeds from this offering. Accordingly, we will retain broad discretion over the use of such proceeds.

The following table sets forth our cash and cash equivalents and our consolidated capitalization as of December 31, 2013 (i) on an actual basis, and (ii) on an as adjusted basis to give effect to the issuance of the securities offered hereby, assuming no exercise of the warrants.

You should read the following table in conjunction with the section entitled “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and our financial statements and notes included in our Annual Report on Form 10-K for the year ended December 31, 2013 which is incorporated by reference in this prospectus supplement and the accompanying prospectus. See “Incorporation of Certain Documents by Reference” and “Where You Can Find More Information.”

If you invest in our common stock and warrants, your ownership interest will be diluted to the extent of the difference between the public offering price per share of common stock that you pay and the as adjusted net tangible book value per share of our common stock immediately after this offering. Net tangible book value per share represents the amount of our total tangible assets less total liabilities, divided by the number of shares of common stock outstanding. Dilution in the as adjusted net tangible book value per share represents the difference between the amount per share of common stock paid by purchasers in this offering and the as adjusted net tangible book value per share of common stock immediately after the consummation of this offering.

As of December 31, 2013, our historical net tangible book value was approximately $(3.9) million, or $(0.42) per share. After giving effect to this offering and the receipt of $6.1 million of estimated net proceeds from this offering, our as adjusted net tangible book value as of December 31, 2013 would have been approximately $2.2 million, or approximately $0.19 per share. This represents an immediate increase in net tangible book value of $0.62 per share to existing stockholders and an immediate and substantial dilution in net tangible book value of $3.22 per share to new investors participating in this offering. The following table illustrates this per share dilution:

The discussion and table above exclude, as of December 31, 2013, unless otherwise noted:

Our common stock is traded on the NASDAQ Capital Market under the symbol “CDTI.” For a twenty-trading day period immediately following the completion of our business combination with CSI and the one-for-six reverse stock split, both of which took effect October 15, 2010, it temporarily traded under the symbol “CDTID” in accordance with NASDAQ’s rules.

The following table sets forth the high and low trading prices of our common stock on the NASDAQ Capital Market for each of the periods listed. Prices indicated below with respect to our share price include inter-dealer prices, without retail mark up, mark down or commission and may not necessarily represent actual transactions.

On March 31, 2014 the last reported sale price for our common stock as quoted on the NASDAQ Capital Market was $3.80 per share.

At March 25, 2014, there were 117 holders of record of our common stock, which excludes stockholders whose shares were held by brokerage firms, depositories and other institutional firms in “street name” for their customers.

We plan to retain any earnings for the foreseeable future for our operations. We have never paid any dividends on our common stock and do not anticipate paying any cash dividends in the foreseeable future. Any future determination to pay cash dividends will be at the discretion of our board of directors and will depend on our financial condition, operating results, capital requirements and such other factors as our board of directors deems relevant. In addition, our credit facility restricts our ability to pay dividends.

In this offering, we are offering units, consisting of 2,030,000 shares of common stock and warrants to purchase up to 812,000 shares of common stock. Each unit consists of one share of common stock and 0.4 of one five and a half-year warrant that is exercisable beginning after the date occurring six months from the date of issuance to purchase one share of common stock at an exercise price of $4.20 per share. The shares of common stock and warrants will be issued separately but can only be purchased together in this offering. This prospectus supplement also relates to the offering of shares of our common stock upon exercise, if any, of the warrants.

The material terms and provisions of our common stock are described under the caption “Description of Common Stock” starting on page 4 of the accompanying prospectus.

The following summary of certain terms and provisions of warrants that are being offered hereby is not complete and is subject to, and qualified in its entirety by the provisions of the warrant, the form of which will be included as an exhibit to a Current Report on Form 8-K to be filed by us with the SEC in connection with this offering. Prospective investors should carefully review the terms and provisions of the form of warrant for a complete description of the terms and conditions of the warrants.

Duration and Exercise Price. The warrants offered hereby will entitle the holders thereof to purchase up to an aggregate of 812,000 shares of common stock at an initial exercise price per share of $4.20, commencing after six months from the date of issuance, and will expire five and a half years from the date of issuance. The warrants will be issued separately from the common stock, and may be transferred separately immediately thereafter. Warrants will be issued in certificated form only. The exercise price is subject to appropriate adjustment in the event of (1) payment of a common stock dividend or other distribution on any class of capital stock that is payable in shares of common stock; (2) subdivisions of outstanding shares of common stock into a larger number of shares; or (3) combinations of outstanding shares of common stock into a smaller number of shares.

Exercisability. The warrants will be exercisable, at the option of each holder, in whole or in part, by delivering to us a duly executed exercise notice accompanied by payment in full for the number of shares of common stock purchased upon such exercise. Only if the registration statement of which this prospectus forms a part, or other applicable registration statement, covering the issuance of the shares of common stock issuable upon exercise of the warrants is not available for such issuance, the holder may exercise the warrant in whole or in part on a cashless basis.

Transferability. Subject to applicable laws and any restrictions on transfer set forth in the warrant, warrants may be transferred at the option of the holder upon surrender of the warrants to us together with the appropriate instruments of transfer.

Exchange Listing. There is no established public trading market for the warrants, and we do not expect a market to develop. We do not intend to apply to list the warrants on any securities exchange. Without an active market, the liquidity of the warrants will be limited. In addition, in the event the price of our common stock does not exceed the per share exercise price of the warrants during the period when the warrants are exercisable, the warrants will not have any value.

Pro Rata Distributions. If, at any time while the warrants are outstanding, we distribute to our common stockholders for no consideration (1) evidences of our indebtedness, (2) any security (other than payment of a common stock dividend or other distribution on any class of capital stock that is payable in shares of common stock), (3) rights or warrants to subscribe for or purchase any security, or (4) any other asset (including cash)

(in each case, “Distributed Property”), then each holder of a warrant shall receive with respect to the shares of common stock issuable upon exercise of such warrant the Distributed Property that such holder would have been entitled to receive had the holder been the record holder of such number of shares of common stock issuable upon exercise of the warrant immediately prior to the record date for such Distributed Property.

Fundamental Transactions. If, at any time while the warrants are outstanding, (1) we consolidate or merge with or into another entity in which the Company is not the surviving entity; (2) we sell all or substantially all of our assets, other than a sale of our engine control systems division or catalyst division or components thereof; (3) any tender offer or exchange offer (whether completed by us or a third party) is completed pursuant to which holders of a majority of the outstanding shares of our common stock tender or exchange their shares for securities, cash or other property; (4) we effect any reclassification of our common stock or compulsory share exchange pursuant to which outstanding common stock is effectively converted or exchange for other securities, cash or property or (5) any transaction is consummated whereby any person or entity acquires more than 50% of the Company’s outstanding common stock (each, a “Fundamental Transaction”), then upon any subsequent exercise of a warrant, the holder thereof will have the right to receive the same amount and kind of securities, cash or other property as it would have been entitled to receive upon the occurrence of such Fundamental Transaction if it had been, immediately prior to such Fundamental Transaction, the holder of the number of shares then issuable upon exercise of the warrant (the “Alternate Consideration”). The Company shall not effect any such Fundamental Transaction unless any successor to the Company, surviving entity or the corporation purchasing or otherwise acquiring such assets or other appropriate corporation or entity shall assume the obligation to deliver to the holders of the warrants such Alternate Consideration as the holders of the warrants may be entitled to purchase. However, each warrant holder may require the Company or any successor entity to pay the warrant holder within 90 days of the Fundamental Transaction an amount of cash equal to the Black Scholes Option Pricing Model obtained from the “OV” function on Bloomberg as of the day immediately following the public announcement of the applicable Fundamental Transaction.

Exercise Limitation. A holder may not exercise its warrants if, after giving effect to the exercise, the holder and certain related parties would beneficially own more than 4.99% of our common stock. A holder may increase or decrease that limitation up to a maximum of 9.99% of our common stock upon not less than 61 days’ prior notice to us.

Right as a Shareholder. The holders of the warrants do not generally have the rights or privileges of holders of our common stock, including any voting rights, until they exercise their warrants.

Waivers and Amendments. Except as otherwise provided in the warrants, we may not may amend or fail to observe the terms of the warrants without the consent of the applicable holder of each such warrant.

Roth Capital Partners, LLC and Craig-Hallum Capital Group LLC, which we refer to as the placement agents, have agreed to act as the exclusive placement agents in connection with this offering subject to the terms and conditions of a placement agent agreement, dated April 1, 2014. The placement agents may engage selected dealers to assist in the placement of the shares and warrants. The placement agents are not purchasing or selling any shares and warrants offered by this prospectus supplement and the related prospectus, nor are they required to arrange the purchase or sale of any specific number or dollar amount of the shares and warrants, but have agreed to use their commercially reasonable “best efforts” to arrange for the sale of all of the shares and warrants offered hereby. We will enter into subscription agreements directly with investors in connection with this offering and we may not sell the entire amount of shares offered pursuant to this prospectus supplement and the related prospectus. The public offering price of the shares and warrants offered hereby has been determined based upon arm’s-length negotiations between the purchasers and us.

The placement agents propose to arrange for the sale to one or more purchasers of the shares and warrants offered pursuant to this prospectus supplement and the related prospectus through direct subscription agreements between the purchasers and us.

We have agreed to pay the placement agents an aggregate cash placement fee equal to 7% of the gross proceeds in this offering.

The following table shows the combined per share and total cash placement agents’ fees we will pay to the placement agents in connection with the sale of the shares and warrants offered pursuant to this prospectus supplement and the related prospectus assuming the purchase of all of the shares and warrants offered hereby:

Because there is no minimum offering amount required as a condition to closing in this offering, the actual total placement agent fees, if any, are not presently determinable and may be substantially less than the maximum amount set forth above. Regardless of whether this offering is consummated, we have agreed to reimburse the placement agent for their reasonable out-of-pocket expenses, including the fees and disbursements of the placement agents’ counsel, up to an amount not to exceed $75,000.

Our obligation to issue and sell shares and warrants to the purchasers is subject to the conditions set forth in the subscription agreements, which may be waived by us at our discretion. A purchaser’s obligation to purchase shares and warrants is subject to the conditions set forth in his or her subscription agreement as well, which may also be waived.

We currently anticipate that the sale of the shares and warrants will be completed on or about April 4, 2014. We estimate the total offering expenses of this offering that will be payable by us, excluding the placement agents’ fees, will be approximately $280,000, which includes legal and printing costs, various other fees and reimbursement of the placements agents’ expenses. At the closing, The Depository Trust Company will credit the shares of common stock to the respective accounts of the purchasers. We will mail warrants directly to the investors at the respective addresses set forth in their subscription agreement with us.

We have agreed to indemnify the placement agents against liabilities under the Securities Act of 1933, as amended. We have also agreed to contribute to payments the placement agents may be required to make in respect of such liabilities.

We, and our executive officers and directors have agreed, subject to certain exceptions, for a period of 90 days after the date of this prospectus supplement, not to offer, sell, contract to sell, pledge, grant any option to purchase, make any short sale or otherwise dispose of, directly or indirectly any shares of common stock or any securities convertible into or exchangeable for shares of our common stock either owned as of the date hereof or thereafter acquired without the prior written consent of Roth Capital Partners LLC. Roth Capital Partners LLC may, in its sole discretion and at any time or from time to time before the termination of the lock-up period, without notice, release all or any portion of the securities subject to lock-up agreement.

This 90-day period may be extended if (1) during the last 17 days of the 90-day period, we issue an earnings release or material news or a material event regarding us occurs or (2) prior to the expiration of the 90-day period, we announce that we will release earnings results during the 16-day period beginning on the last day of the 90-day period, then the period of such extension will be 18 days, beginning on the issuance of the earnings release or the occurrence of the material news or material event.

This prospectus supplement and the related prospectus may be made available in electronic format on websites or through other online services maintained by the placement agent, or by an affiliate. Other than this prospectus supplement and the related prospectus, the information on the placement agent’s website and any information contained in any other website maintained by the placement agent is not part of this prospectus supplement and the related prospectus or the registration statement of which this prospectus supplement and the related prospectus forms a part, has not been approved and/or endorsed by us or the placement agent, and should not be relied upon by investors.

The foregoing does not purport to be a complete statement of the terms and conditions of the placement agency agreement and subscription agreements. A copy of the placement agent agreement and the form of subscription agreement with the purchasers are included as exhibits to our current report on Form 8-K that will be filed with the SEC and incorporated by reference into the Registration Statement of which this prospectus supplement forms a part. See “Where You Can Find Additional Information.”

The placement agents may be deemed to be underwriters within the meaning of Section 2(a)(11) of the Securities Act, and any commissions received by them and any profit realized on the resale of the shares and warrants sold by them while acting as a principal might be deemed to be underwriting discounts or commissions under the Securities Act. As underwriters, the placement agents would be required to comply with the requirements of the Securities Act and the Securities Exchange Act of 1934, as amended, including, without limitation, Rule 415(a)(4) under the Securities Act and Rule 10b-5 and Regulation M under the Exchange Act. These rules and regulations may limit the timing of purchases and sales of shares and warrants by the placement agents acting as a principal. Under these rules and regulations, the placement agents:

Our common stock is listed on the NASDAQ Capital Market and trades under the symbol “CDTI” The transfer agent of our common stock is American Stock Transfer & Trust Company, LLC. The transfer agent’s address is 6201 15^th Avenue, Third Floor, Brooklyn, New York 11219, and its telephone number is: (800) 937-5449.

There is no established public trading market for the warrants and we do not expect a market to develop. In addition, we do not intend to apply for listing of the warrants on any national securities exchange. The Company will act as warrant agent with respect to the warrants offered hereby.

The placement agents and/or their respective affiliates have provided, and may in the future provide, various investment banking and other financial services for us for which services they have received and, may in the future receive, customary fees. Except for services provided in connection with this offering, neither of the placement agents has provided any investment banking or other financial services during the 180-day period preceding the date of this prospectus supplement and we do not expect to retain either of the placement agents to perform any investment banking or other financial services for at least 90 days after the date of this prospectus supplement.

The validity of the shares of common stock and warrants offered hereby will be passed upon for us by DLA Piper LLP (US), Phoenix, Arizona. The placement agents have been represented in connection with this offering by Loeb & Loeb LLP, New York, New York.

The consolidated financial statements of the Company as of and for the years ended December 31, 2013 and 2012 have been incorporated by reference in this prospectus supplement in reliance upon the report of BDO USA, LLP, independent registered public accounting firm, incorporated by reference herein, and upon the authority of said firm as an expert in accounting and auditing.

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.

Upon request, we will provide each person to whom this prospectus supplement is delivered a copy of all of the information that has been incorporated by reference in this prospectus supplement or the accompanying prospectus but not delivered with this prospectus supplement and the accompanying prospectus. You may obtain copies of 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:

The SEC allows us to “incorporate by reference” into this prospectus supplement 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 supplement and the accompanying prospectus, and later information that we file with the SEC will update and supersede this information.

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

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), until the offering of all the securities by this prospectus supplement is completed, including all filings made after the date of this prospectus supplement. 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 supplement, 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.

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 3 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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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:

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.

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.

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.

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.

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.

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.

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.

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.”

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:

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.

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.

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

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

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.

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.

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

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.”

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.

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

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.

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.

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.

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.

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.”

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.

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

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.

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.

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

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:

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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:

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:

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.


	Per Share and corresponding Warrant		Total
Public Offering Price	$	3.40	$	6,902,000
Placement Agent Fee (1)	$	0.24	$	483,140
Proceeds to CDTI (before expenses)	$	3.16	$	6,418,860


Prospectus Supplement

ABOUT THIS PROSPECTUS SUPPLEMENT			S-1
PROSPECTUS SUPPLEMENT SUMMARY			S-2
RISK FACTORS			S-8
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS			S-14
USE OF PROCEEDS			S-15
CAPITALIZATION			S-16
DILUTION			S-17
MARKET PRICE AND DIVIDEND INFORMATION			S-18
DESCRIPTION OF SECURITIES			S-19
PLAN OF DISTRIBUTION			S-21
LEGAL MATTERS			S-24
EXPERTS			S-24
WHERE YOU CAN FIND MORE INFORMATION			S-24
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE			S-25

Prospectus

ABOUT THIS PROSPECTUS			1
ABOUT OUR COMPANY			1
RISK FACTORS			3
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS			3
USE OF PROCEEDS			3
DILUTION			4
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			15
EXPERTS			15
WHERE YOU CAN FIND MORE INFORMATION			15
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE			16


	As of December 31, 2013
	Actual			As Adjusted (1)
	(In thousands)
Cash	$	3,909		$	10,048
Borrowings
Line of credit		2,258			2,258
8% shareholder note due 2015		1,586			1,586
8% subordinated convertible shareholder notes due 2016		3,000			3,000
8% shareholder note due 2015		2,963			2,963
Total borrowings		9,807			9,807
Stockholders’ equity
Preferred stock, par value $0.01 per share: authorized 100,000; no shares issued and outstanding		0			0
Common stock, par value $0.01 per share: authorized 24,000,000; 9,290,186 shares issued and outstanding		93			113
Additional paid-in capital		188,108			194,227
Accumulated other comprehensive loss		(1,036	)		(1,036	)
Accumulated deficit		(181,719	)		(181,719	)
Total stockholders’ equity		5,446			11,585
Total capitalization	$	19,162		$	31,440	(2)


	NASDAQ Capital Market
	High		Low
2012
1^st Quarter	$	5.28	$	2.69
2^nd Quarter	$	4.20	$	1.93
3^rd Quarter	$	3.35	$	2.00
4^th Quarter	$	3.04	$	2.00
2013
1^st Quarter	$	3.05	$	2.10
2^nd Quarter	$	2.27	$	1.15
3^rd Quarter	$	2.08	$	1.10
4^th Quarter	$	1.86	$	1.31
2014
1^st Quarter	$	7.39	$	1.51


Per Share of Common Stock and corresponding Warrant	$	0.24
Total	$	483,140


Public offering price per share of common stock and corresponding warrant	$	3.40
Historical net tangible book value per share as of December 31, 2013	$	(0.42	)
Increase in net tangible book value per share attributable to this offering	$	0.62
Adjusted net tangible book value per share after giving effect to this offering	$	0.19
Dilution per share to new investors	$	3.21


	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	$	—



Roth Capital Partners		Craig-Hallum Capital Group