Exhibit 5.1

                    [Simpson Thacher & Bartlett Letterhead]



                                       September 24, 1998



Clark Refining & Marketing, Inc.
8182 Maryland Avenue
St. Louis, Missouri 63105

Ladies and Gentlemen:

     We have acted as special counsel for Clark Refining & Marketing, Inc., a
Delaware corporation (the "Company"), in connection with the Registration
Statement on Form S-4 (the "Registration Statement") filed by the Company with
the Securities and Exchange Commission (the "Commission") under the Securities
Act of 1933, as amended (the "Securities Act"), relating to the issuance by the
Company of $110,000,000 aggregate principal amount of its 8 5/8% New Senior
Notes due 2008 (the "New Notes"). The New Notes are to be offered by the Company
in exchange (the "Exchange") for $110,000,000 aggregate principal amount of its
outstanding 8 5/8% Senior Notes due 2008 (the "Old Notes"). The Old Notes have
been, and the News Notes will be, issued under an Indenture dated as of August
10, 1998 (the "Indenture"), between the Company and Bankers Trust Company, as
Trustee (the "Trustee").

     We have examined the Registration Statement and the Indenture which has
been filed with the Commission as an Exhibit to the Registration Statement. In
addition, we have examined, and have relied as to matters of fact upon, the
originals or copies, certified

 
Clark Refining &
Marketing, Inc.                       -2-                     September 24, 1998
                                      
or otherwise identified to our satisfaction, of such corporate records,
agreements, documents and other instruments and such certificates or comparable
documents of public officials and of officers and representatives of the
Company, and have made such other and further investigations, as we have deemed
relevant and necessary as a basis for the opinion hereinafter set forth.

     In such examination, we have assumed the genuineness of all signatures, the
legal capacity of natural persons, the authenticity of all documents submitted
to us as originals and the conformity to original documents of all documents
submitted to us as certified or photostatic copies, and the authenticity of the
originals of such latter documents.

     Based upon the foregoing, and subject to the qualifications and limitations
stated herein, assuming that the Indenture is the valid and legally binding
obligation of the Trustee, when (i) the Indenture has been duly qualified under
the Trust Indenture Act of 1939, as amended (the "Trust Indenture Act") and (ii)
the New Notes have been duly executed, authenticated, issued and delivered in
accordance with the provisions of the Indenture upon the Exchange, we are of the
opinion that the New Notes will constitute valid and legally binding obligations
of the Company, enforceable against the Company in accordance with their terms.

     Our opinion set forth in the preceding sentence is subject to the effects
of bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium and
other similar laws relating to or affecting creditors' rights generally, general
equitable principles (whether considered in a proceeding in equity or at law)
and an implied covenant of good faith and fair dealing.

     We are members of the Bar of the State of New York and we do not express

 
Clark Refining &
Marketing, Inc.                       -3-                     September 24, 1998

any opinion herein concerning any law other than the law of the State of New
York.

     We hereby consent to the use of this opinion as an Exhibit to the
Registration Statement and to the reference to our firm under the caption "Legal
Matters" in the Prospectus included therein.

                                 Very truly yours,
 

                                 SIMPSON THACHER & BARTLETT