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


               [On Cleary, Gottlieb, Steen & Hamilton letterhead]



                                 August 28, 2003


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. 2003-13 (the "Trust"), the issuance by the
Trust of $30,000,000 principal amount of Class A Units (the "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 August 25, 2003, and 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 August 28, 2003 (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 August 28, 2003 (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