Exhibit 5.1

Re:                               A123 Systems, Inc. Offering of Common Stock and Warrants

Ladies and Gentlemen:

We have acted as special counsel to A123 Systems, Inc., a Delaware corporation (the “Company”), in connection with the sale to the purchasers (the “Purchasers”) of 7,692,308 shares (the “Shares”) of common stock of the Company, par value $0.001 per share (the “Common Stock”) and (ii) warrants to purchase additional shares of Common Stock (the “Warrants”), pursuant to (i) a registration statement on Form S-3/A under the Securities Act of 1933, as amended (the “Act”), filed with the Securities and Exchange Commission (the “Commission”) on June 22, 2012 (File No. 333-173122), including the information deemed to be a part thereof pursuant to Rule 430B under the Act (as so filed and as amended, the “Registration Statement”), (ii) a base prospectus, dated June 27, 2012 (the “Base Prospectus”), (iii) a preliminary prospectus supplement, dated July 6, 2012 (the “Preliminary Prospectus Supplement” and, together with the Base Prospectus, the “Preliminary Prospectus”), (iv) a prospectus supplement, dated July 6, 2012, filed with the Commission pursuant to Rule 424(b) under the Act (the “Prospectus Supplement” and, together with the Base Prospectus, the “Prospectus”) and (v) the subscription agreements, each dated as of July 5, 2012, between the Purchasers and the Company (the “Subscription Agreements”).  This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement, Preliminary Prospectus or Prospectus, other than as expressly stated herein with respect to the issue of the Shares and Warrants.

As such counsel, we have examined such matters of fact and questions of law as we have considered appropriate for purposes of this letter.  With your consent, we have relied upon certificates and other assurances of officers of the Company and others as to factual matters without having independently verified such factual matters.  We are opining herein as to the General Corporation Law of the State of Delaware (the “DGCL”) and, with respect to paragraph 2, the internal laws of the State of New York, and we express no opinion with respect to the applicability thereto, or the effect thereon, of the laws of any other jurisdiction or, in the case of the State of Delaware, any other laws, or as to any matters of municipal law or the laws of any local agencies within any state.

Subject to the foregoing and the other matters set forth herein, it is our opinion that, as of the date hereof:

1.                                       When the Shares shall have been duly registered on the books of the transfer agent and registrar therefor in the name or on behalf of the Purchasers and have been issued by the Company against payment therefor (not less than par value) in the circumstances contemplated by the Subscription Agreements, the issue and sale of the Shares will have been duly authorized by all necessary corporate action of the Company, and the Shares will be validly issued, fully paid and nonassessable.

2.                                       When the Warrants have been duly executed, authenticated, issued and delivered against payment therefor in the circumstances contemplated by the Subscription Agreements, the issue and sale of the Warrants will have been duly authorized by all necessary corporate action of the Company, and the Warrants will be legally valid and binding obligations of the Company, enforceable against the Company in accordance with their terms.

3.                                       When the shares of Common Stock initially issuable upon exercise of the Warrants (the “Warrant Shares”) shall have been duly registered on the books of the transfer agent and registrar therefor in the name or on behalf of the Warrant holders, and have been issued by the Company against payment therefor (not less than par value) in the circumstances contemplated by the Warrants, the issue of the Warrant Shares will have been duly authorized by all necessary corporate action of the Company, and the Warrant Shares will be validly issued, fully paid and nonassessable

Our opinions are subject to: (i) the effect of bankruptcy, insolvency, reorganization, fraudulent transfer, moratorium or other similar laws relating to or affecting the rights and remedies of creditors; and (ii) the effect of general principles of equity, whether considered in a proceeding in equity or at law (including the possible unavailability of specific performance or injunctive relief), concepts of materiality, reasonableness, good faith and fair dealing, and the discretion of the court before which a proceeding is brought.  We express no opinion or confirmation as to federal or state securities laws, tax laws, antitrust or trade regulation laws, insolvency or fraudulent transfer laws, antifraud laws, compliance with fiduciary duty requirements, pension or employee benefit laws, FINRA rules or stock exchange rules (without limiting other laws excluded by customary practice).

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act.  We consent to your filing this opinion as an exhibit to the Company’s Current Report on Form 8-K dated July 10, 2012 and to the reference to our firm in the Prospectus under the heading “Legal Matters.”  In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder.

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