Exhibit 5.1

                                   [UNOCAL LETTERHEAD]








                                   July 15, 2005



Unocal Corporation
2141 Rosecrans Avenue
Suite 4000
El Segundo, California  90245

         Re:      Unocal Corporation:
                  Unocal Deferred Compensation Plan of 2005


Ladies and Gentlemen:

         I am the Senior Vice President, Chief Legal Officer and General Counsel
of Unocal Corporation, a Delaware corporation ("Unocal"), and as such I, and
attorneys working for me, have acted on behalf of Unocal as counsel in
connection with the preparation of the Registration Statement on Form S-8 (the
"Registration Statement"), which Unocal proposes to file with the Securities and
Exchange Commission. The Registration Statement relates to the registration
under the Securities Act of 1933, as amended (the "Securities Act"), of
$40,000,000 of Deferred Compensation Obligations of Unocal (the "Obligations"),
to be issued pursuant to the Unocal Deferred Compensation Plan of 2005 (the
"Plan").

         This opinion is rendered in accordance with the requirements of Item
601(b)(5) of Regulation S-K of the Securities and Exchange Commission.

         I, or such attorneys working for me, have examined or considered such
matters of law and fact and such corporate records, certificates and other
documents as I, or they, have deemed necessary, including, but not limited to:
the Certificate of Incorporation and Bylaws of Unocal, as amended to date;
certain resolutions adopted by the Board of Directors of Unocal at a meeting
duly called and held; and the Plan. In the course of such examination, I, and
such attorneys working for me, have assumed the genuineness of all signatures,
the authenticity of all documents submitted as originals and the conformity to
the originals of all documents submitted as certified, photostatic or conformed
copies. I have relied, as to certain legal matters, on the advice of such
attorneys working for me who are more familiar with such matters.

         I am licensed to practice law in the state of New York and am familiar
with the Delaware General Corporation Law, the Delaware Constitution and
reported judicial decisions interpreting these laws. Therefore, the following
opinions are limited to the Delaware General Corporation




Law and the Delaware Constitution and reported judicial decisions interpreting
these laws, and the federal laws of the United States, to the exclusion of all
other jurisdictions.

         Based upon such examination and upon such matters of fact and law as I
have deemed relevant, I am of the opinion that:

          1.   The  Obligations  have  been  duly  authorized  by all  necessary
               corporate action on the part of Unocal;

          2.   When  incurred  in  accordance  with  such   authorization,   the
               provisions of the Plan and relevant elections and agreements duly
               authorized by and in accordance  with the terms of the Plan,  the
               Obligations  will be legally  valid and  binding  obligations  of
               Unocal,  enforceable  against  Unocal in  accordance  with  their
               terms,  except  as  may be  limited  by  bankruptcy,  insolvency,
               reorganization,   moratorium  or  similar  laws  relating  to  or
               affecting   creditors'  rights  generally   (including,   without
               limitation,   fraudulent   conveyance   laws),   and  by  general
               principles of equity including,  without limitation,  concepts of
               materiality,  reasonableness, good faith and fair dealing and the
               possible  unavailability  of specific  performance  or injunctive
               relief,  regardless  of whether  considered  in a  proceeding  in
               equity or at law; and

         I hereby consent to the filing of this opinion with the Securities and
Exchange Commission as an Exhibit to the Registration Statement and to the
reference to me under the caption "Interests of Named Experts and Counsel"
therein. In giving this consent, I do not thereby admit that I am included in
the category of persons whose consent is required under Section 7 of the
Securities Act or the rules and regulations of the Commission promulgated
thereunder.

                                            Sincerely,

                                            /s/Samuel H. Gillespie III