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                                                                     EXHIBIT 8.1

                                  April 7, 2000



Re: REGISTRATION STATEMENT ON FORM S-4

Quintus Corporation
47212 Mission Falls Court
Fremont, CA 94539

Ladies and Gentlemen:

        We have acted as counsel for Quintus Corporation ("PARENT"), a Delaware
corporation, in connection with (i) the Merger (the "MERGER"), as defined and
described in the Agreement and Plan of Merger dated as of February 25, 2000 (the
"MERGER AGREEMENT") by and between Mustang.com, Inc. ("COMPANY"), a California
corporation, and Parent and (ii) the preparation and filing of the related
Registration Statement on Form S-4, which includes the Proxy
Statement/Prospectus (the "PROXY STATEMENT/ PROSPECTUS"), filed with the
Securities and Exchange Commission (the "COMMISSION"). You have requested our
opinion regarding the United States federal income tax consequences of the
Merger. Unless otherwise indicated, each capitalized term used herein has the
meaning ascribed to it in the Merger Agreement.

        In connection with this opinion, we have examined the Merger Agreement,
the Proxy Statement/Prospectus and such other documents as we have deemed
necessary or appropriate in order to enable us to render our opinion. For
purposes of this opinion, we have assumed (i) the validity and accuracy of the
documents that we have examined, (ii) that the Merger will be consummated in the
manner described in Merger Agreement and the Proxy Statement/Prospectus, and
(iii) that the representations made by Parent (together with Merger Subsidiary)
and Company pursuant to Sections 8.02(f) and 8.03(b), respectively, of the
Merger Agreement are accurate and complete. In rendering our opinion, we have
considered the applicable provisions of the Internal Revenue Code of 1986, as
amended (the "CODE"), Treasury Department regulations promulgated thereunder,
pertinent judicial authorities, interpretive rulings of the Internal Revenue
Service and such other authorities as we have considered relevant. It should be
noted that statutes, regulations, judicial decisions and administrative
interpretations are subject to change at any time (possibly with retroactive
effect). A change in the authorities or the inaccuracy of any of the documents
or assumptions on which our opinion is based could affect our conclusions.



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        Based upon the foregoing, in our opinion, the Merger will be treated for
United States federal income tax purposes as a reorganization within the meaning
of Section 368(a) of the Code, and Parent, Company and Merger Subsidiary will
each be a party to that reorganization within the meaning of Section 368(b) of
the Code. Other than as expressly set forth above, we express no opinion as to
the United States federal, state, local, foreign or other tax consequences of
the Merger.

        We hereby consent to the discussion of this opinion in the Proxy
Statement/Prospectus, to the filing of this opinion as an exhibit to the Proxy
Statement/Prospectus and to the reference to our firm under the headings "THE
MERGER-Material United States Federal Income Tax Consequences" and "LEGAL
MATTERS" in the Proxy Statement/Prospectus. In giving such consent, we do not
thereby admit that we are in the category of persons whose consent is required
under Section 7 of the Securities Act of 1933, as amended, or the rules and
regulations of the Commission thereunder.

                                                   Very truly yours,

                                                   /s/ Davis Polk & Wardwell