EXHIBIT 5.1

[Letterhead of Pepper Hamilton]

June 20, 2003

Neose Technologies, Inc.

102 Witmer Road

Horsham, PA 19044

Ladies and Gentlemen:

This opinion is furnished to you in connection with the Registration Statement on Form S-3 (the “Registration Statement”) being filed by Neose Technologies, Inc., a Delaware corporation (the “Company”), with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), with respect to shares of the Company’s common stock, $0.001 par value per share (the “Common Stock”) which may be issued from time to time on a delayed or continuous basis pursuant to Rule 415 under the Securities Act at an aggregate initial offering price not to exceed $75,000,000.

We have examined the Registration Statement, including the exhibits thereto, the Company’s Certificate of Incorporation and Bylaws, as in effect on the date hereof, certain resolutions of the Board of Directors of the Company and such other documents, corporate records, and instruments and have examined such laws and regulations as we have deemed necessary for purposes of rendering the opinions set forth herein. In our examination, we have assumed the legal capacity of all natural persons, the genuineness of all signatures, the authenticity of all documents submitted to us as certified or photostatic copies and the authenticity of the originals of such latter documents. As to any facts material to the opinions expressed herein which were not independently established or verified, we have relied upon statements and representations of officers and other representatives of the Company and others.

Members of our firm are admitted to the Bar of the Commonwealth of Pennsylvania and we express no opinion as to the laws of any other jurisdiction other than the Federal laws of the United States of America and the General Corporation Law of the State of Delaware, including the statutory provisions and all applicable provisions of the Delaware Constitution and reported judicial decisions interpreting these laws, each as in effect on the date hereof.

Based upon such examination and subject to the further provisions hereof, we are of the opinion that with respect to the Common Stock, when (i) specifically authorized for issuance by the Company’s Board of Directors or an authorized committee thereof (the “Authorizing Resolutions”), (ii) the Registration Statement has become effective under the Securities Act, (iii) the terms of the sale of the Common Stock have been duly established in conformity with the Company’s Certificate of Incorporation and By-laws and do not violate any applicable law or result in a default under or breach of any agreement or instrument binding on

Neose Technologies, Inc.

June 20, 2003

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the Company and comply with any requirement or restriction imposed by any court or governmental body having jurisdiction over the Company, (iv) the Common Stock has been issued and sold as contemplated by the Registration Statement, and (v) the Company has received the consideration provided for in the Authorizing Resolutions, the Common Stock will be validly issued, fully paid and nonassessable.

It is understood that this opinion is to be used only in connection with the offer and sale of Common Stock while the Registration Statement is in effect.

Please note that we are opining only as to the matters expressly set forth herein, and no opinion should be inferred as to any other matters. This opinion is based upon currently existing statutes, rules, regulations and judicial decisions, and we disclaim any obligation to update this opinion or otherwise advise you of any change in any of these sources of law or subsequent legal or factual developments which might affect any matters or opinions set forth herein. An additional legal opinion shall be issued and filed with the Commission upon the issuance of shares of Common Stock in accordance with the terms of the Registration Statement.

We hereby consent to the filing of this opinion with the Commission as an exhibit to the Registration Statement in accordance with the requirements of Item 601(b) (5) of Regulation S-K under the Securities Act and to the use of this Firm’s name therein and in the related prospectus under the caption “Legal Matters.” In giving such consent, we do not hereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission.

Very truly yours,

/s/ Pepper Hamilton LLP

PEPPER HAMILTON LLP