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                                                                     Exhibit 8.1

                                                                October 10, 2001

Washington Mutual, Inc.
1201 Third Avenue
Seattle, Washington 98101

        Re:     Agreement and Plan of Merger dated as of June 25, 2001, between
                Washington Mutual, Inc. and Dime Bancorp, Inc.

Ladies and Gentlemen:

        We have acted as special counsel to Washington Mutual, Inc., a
Washington corporation ("Washington Mutual"), in connection with the proposed
merger (the "Merger") of Dime Bancorp, Inc., a Delaware corporation ("Dime"),
with and into Washington Mutual with the separate corporate existence of Dime
ceasing and Washington Mutual continuing as the surviving corporation. The
Merger will be consummated pursuant to the Agreement and Plan of Merger dated as
of June 25, 2001, as amended or supplemented through the date hereof, between
Washington Mutual and Dime (the "Merger Agreement"). The time at which the
Merger becomes effective is hereafter referred to as the "Effective Time." For
purposes of this opinion, capitalized terms used and not otherwise defined
herein shall have the meaning ascribed thereto in the Merger Agreement. This
opinion is being delivered in connection with Washington Mutual's Registration
Statement on Form S-4 relating to the proposed Merger (the "Registration
Statement") to which this opinion appears as an exhibit.

        In acting as counsel to Washington Mutual in connection with the Merger,
we have, in preparing our opinion, as hereinafter set forth, participated in the
preparation of the Merger Agreement and the preparation and filing of the
Registration Statement.

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        You have requested that we render the opinion set forth below. In
rendering such opinion, we have assumed with your consent that (i) the Merger
will be effected in accordance with the Merger Agreement, (ii) the statements
concerning the Merger set forth in the Merger Agreement and the Registration
Statement are true, complete and correct and will remain true, complete and
correct at all times up to and including the Effective Time, (iii) the
representations made by Washington Mutual and Dime in their respective letters
delivered to us for purposes of this opinion (the "Representation Letters") are
true, complete and correct and will be true, complete and correct at all times
up to and including the Effective Time as if made as of the Effective Time, and
(iv) any representations made in the Representation Letters "to the best
knowledge of" or similarly qualified are true, correct and complete and will
remain true, correct and complete at all times up to and including the Effective
Time, in each case without such qualification. We have also assumed that the
parties have complied with and, if applicable, will continue to comply with, the
covenants contained in the Merger Agreement.

        We have examined the documents referred to above and the originals, or
duplicates or certified or conformed copies, of such records, documents,
certificates or other instruments and made such other inquiries as in our
judgment are necessary or appropriate to enable us to render the opinion set
forth below. 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, the conformity to original documents of
all documents submitted to us as duplicates or certified or conformed copies,
and the authenticity of the originals of such latter documents. We have not,
however, undertaken any independent investigation of any factual matter set
forth in any of the foregoing.

        Our opinion is based on the Internal Revenue Code of 1986, as amended
(the "Code"), Treasury Regulations, administrative interpretations, and judicial
precedents as of the

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date hereof. If there is any subsequent change in the applicable law or
regulations, or if there are subsequently any new applicable administrative or
judicial interpretations of the law or regulations, or if there are any changes
in the facts or circumstances surrounding the Merger, the opinion expressed
herein may become inapplicable.

        Based upon the foregoing, and subject to the qualifications and
limitations stated herein, we hereby confirm our opinion set forth in the
discussion contained in the Registration Statement under the caption "THE MERGER
AGREEMENT --Material United States Federal Income Tax Consequences".

        We express our opinion herein only as to those matters specifically set
forth above and no opinion should be inferred as to the tax consequences of the
Merger under any state, local or foreign law, or with respect to other areas of
United States federal taxation. 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 filing of this opinion as Exhibit 8.1 to the
Registration Statement and to the references to our firm name therein.


                                                     Very truly yours,

                                                     SIMPSON THACHER & BARTLETT