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                   [LETTERHEAD OF SIMPSON THACHER & BARTLETT]



                                                           December 4, 2000


                   RE: Issuance and Sale of Capital Securities


BancWest Corporation
999 Bishop Street
Honolulu, Hawaii 96813

BancWest Capital I
999 Bishop Street
Honolulu, Hawaii 96813

Ladies and Gentlemen:

         We have acted as tax counsel ("Tax Counsel") to BancWest Corporation, a
Delaware corporation (the "Company"), and BancWest Capital I, a statutory
business trust created under the laws of the State of Delaware (the "Trust"), in
connection with the preparation and filing by the Company and the Trust with the
Securities and Exchange Commission (the "Commission") of the Prospectus
Supplement dated December 4, 2000 to the Prospectus dated November 6, 2000
(together, the "Prospectus"), which formed a part of the Registration Statement
on Form S-3 (as amended, the "Registration Statement") under the Securities Act
of 1933, as amended, with respect to (i) the issuance of the Junior Subordinated
Debentures (the "Subordinated Debentures") by the Company pursuant to the
Indenture (the "Indenture") between the Company and Bank One Trust Company,
N.A., as trustee (in such capacity, the "Debenture Trustee") in exchange for
Capital Securities and Common Securities (as defined below) of BancWest Capital
I, in the form filed as an exhibit to the Registration Statement; (ii) the
issuance of the Trust Preferred Securities (the "Capital

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Securities") and Common Securities (the "Common Securities", and together, the
"Trust Securities") in exchange for the Subordinated Debentures pursuant to the
Amended and Restated Declaration of Trust (the "Trust Agreement") among the
Company, as sponsor, Bank One Trust Company, N.A., as Property Trustee, Bank One
Delaware, Inc., as Delaware Trustee and the Administrative Trustees named
therein, in the form filed as an exhibit to the Registration Statement; and
(iii) the sale by the Company of the Capital Securities to investors pursuant to
the Prospectus.

         The Trust Securities are guaranteed by the Company with respect to the
payment of distributions and payments upon liquidation, redemption and otherwise
pursuant to, and to the extent set forth in, the Guarantee Agreement ("the
Guarantee"), between the Company and Bank One Trust Company, N.A., as trustee
(in such capacity, the "Guarantee Trustee") for the benefit of the holders of
the Trust Securities, in the form filed as an exhibit to the Registration
Statement.

         All capitalized terms used in this opinion letter and not otherwise
defined herein shall have the meaning ascribed to such terms in the Registration
Statement.

         In delivering this opinion letter, we have reviewed and relied upon:
(i) the Registration Statement; (ii) a form of the Indenture; (iii) a form of
the Subordinated Debenture; (iv) a form of the Trust Agreement; (v) a form of
the Guarantee; and (vi) forms of the Trust Securities, in the case of each
"form", as such form was filed as an exhibit to the Registration Statement. We
also have examined and relied upon originals, or duplicates or certified or
conformed copies, of such records of the Company and the Trust and such other

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documents, certificates and records as we have deemed necessary or appropriate
as a basis for the opinion set forth herein.

         In our examination of such material, we have assumed the genuineness of
all signatures, the authenticity of all documents submitted to us as originals
and the conformity to original documents of all copies of documents submitted to
us. In addition, we also have assumed that the transaction related to the
issuance of the Subordinated Debentures and the Trust Securities will be
consummated in accordance with the terms of the documents and forms of documents
described herein.

         On the basis of the foregoing and assuming that the Trust was formed
and will be maintained in compliance with the terms of the Trust Agreement, we
hereby confirm our opinion set forth in the Prospectus under the caption "United
States Federal Income Tax Consequences".

         We express no opinion with respect to the transactions referred to
herein or in the Prospectus other than as expressly set forth herein. Moreover,
we note that there is no authority directly on point dealing with securities
such as the Capital Securities or transactions of the type described herein and
that our opinion is not binding on the Internal Revenue Service or the courts,
either of which could take a contrary position. Nevertheless, we believe that
the opinion expressed in the Prospectus, if challenged, would be sustained by a
court with jurisdiction in a properly presented case.

         Our opinion is based upon the Internal Revenue Code of 1986, as
amended, the Treasury Regulations promulgated thereunder and other relevant
authorities and law, all as in effect on the date hereof. Consequently, future
changes in the law may cause the tax

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treatment of the transactions referred to herein to be materially different from
that described above.

         We are members of the Bar of the State of New York, and we do not
express any opinion herein concerning any law other than the federal law of the
United States.

         We hereby consent to the use of our name in the Registration Statement
under the caption "United States Federal Income Tax Consequences" and "Legal
Opinions".



                                         Very truly yours,


                                         /s/ Simpson Thacher & Bartlett


                                         SIMPSON THACHER & BARTLETT