Exhibit 5.1
                                                                     -----------

               [On Cleary, Gottlieb, Steen & Hamilton letterhead]


                                                     April 15, 2004


Morgan Stanley & Co. Incorporated
1585 Broadway
New York, New York 10036

MS Structured Asset Corp.
1585 Broadway
New York, New York 10036

Ladies and Gentlemen:

                  We have acted as special counsel to MS Structured Asset Corp.
(the "Depositor") in connection with the formation of Structured Asset Trust
Unit Repackagings ("SATURNS") Trust No. 2004-6 (the "Trust"), the issuance by
the Trust of $80,000,000 principal amount of Class A Units and the offer and
sale of the Class A Units by the Underwriter. The Class A Units are offered
pursuant to a registration statement on Form S-3 (No. 333-101155). Such
registration statement, as amended when it became effective, but excluding the
documents incorporated by reference therein, is herein called the "Registration
Statement," and the related prospectus, as supplemented by the prospectus
supplement dated April 12, 2004 relating to the Class A Units, as first filed
with the Securities and Exchange Commission pursuant to Rule 424(b)(2) under the
Securities Act, but excluding the documents incorporated by reference therein,
is herein called the "Prospectus." This opinion letter is furnished pursuant to
Section 4(c) of the Underwriting Agreement dated April 15, 2004 (the
"Underwriting Agreement") between the Depositor and the Underwriter. The Trust
is organized under the laws of the State of New York, pursuant to the Trust
Agreement, dated April 15, 2004 (the "Trust Agreement"), between LaSalle Bank
National Association, as trustee (the "Trustee") and the Depositor, which Trust
Agreement, to the extent provided therein, incorporates by reference the
Standard Terms referenced therein. Unless otherwise defined herein, capitalized
terms used herein shall have the meanings ascribed thereto in the Trust
Agreement.

                  In arriving at the opinions expressed below, we have reviewed
the following documents:

                  (a) the Prospectus and the documents incorporated by reference
therein;

                  (b) the Trust Agreement; and

                  (c) the Class A Units.

                  In addition, we have reviewed the originals or copies
certified or otherwise identified to our satisfaction of all such corporate
records of the Depositor and such other instruments and other certificates of
public officials, officers and representatives of the Depositor and such other
persons, and we have made such investigations of law, as we have deemed
appropriate as a basis for the opinions expressed below.

                  In rendering the opinions expressed below, we have assumed the
authenticity of all documents submitted to us as originals and the conformity to
the originals of all documents submitted to us as copies. In addition, we have
assumed and have not verified the accuracy as to factual matters of each
document we have reviewed (including, without limitation, the accuracy of the
representations and warranties of the Depositor in the Standard Terms).

                  Based on the foregoing, and subject to the further assumptions
and qualifications set forth below, it is our opinion that:

                  1. Assuming the Trust Agreement has been duly authorized,
executed and delivered by each of the Trustee and the Depositor, the Trust
Agreement is the valid and binding obligation of the Depositor enforceable
against the Depositor in accordance with its terms.

                  2. The Class A Units, assuming that they have been duly and
validly authorized, executed, and issued by the Trustee, authenticated as
specified in the Trust Agreement and delivered to the Depositor, have been
legally and validly issued, the holders of the Class A Units will be entitled to
the benefits of the Trust Agreement, and the Class A Units are fully paid and
nonassessable.

                  Insofar as the foregoing opinions relate to the validity,
binding effect or enforceability of any agreement or obligation (a) we have
assumed that each other party to such agreement or obligation other than the
Depositor has satisfied those legal requirements that are applicable to it to
the extent necessary to make such agreement or obligation enforceable against
it, and (b) such opinions are subject to applicable bankruptcy, insolvency and
similar laws affecting creditors' rights generally and to general principles of
equity.

                  The foregoing opinions are limited to the federal law of the
United States of America, and the law of the State of New York.

                  We hereby consent to the filing of this opinion as an exhibit
to the Registration Statement and the reference to this firm in the Registration
Statement and the related Prospectus under the caption "Validity of Units". By
giving such consent, we do not admit that we are "experts" within the meaning of
the Act, or the rules and regulations of the Commission issued thereunder with
respect to any part of the Registration Statement, including this exhibit.

                                              Very truly yours,

                                              CLEARY, GOTTLIEB, STEEN & HAMILTON



                                              By  /s/ Michael A. Mazzuchi
                                                  ------------------------------
                                                  Michael A. Mazzuchi, a Partner