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                                                                     EXHIBIT 5.1

                         [GUNDERSON DETTMER LETTERHEAD]

                                October 22, 1999


Quintus Corporation
47212 Mission Falls Court
Fremont, California 94539


                  Re:      Registration Statement on Form S-1

Ladies and Gentlemen:

                  We have examined the Registration Statement on Form S-1 (File
No. 333-86919) originally filed by Quintus Corporation (the "Company") with the
Securities and Exchange Commission (the "Commission") on September 10, 1999, as
thereafter amended or supplemented (the "Registration Statement"), in connection
with the registration under the Securities Act of 1933, as amended, of up to
4,600,000 shares of the Company's Common Stock (the "Shares"). The Shares, which
include an over-allotment option granted by the Company to the Underwriters to
purchase up to 600,000 additional shares of the Company's Common Stock, are to
be sold to the Underwriters by the Company as described in the Registration
Statement for resale to the public. As your counsel in connection with this
transaction, we have examined the proceedings taken and are familiar with the
proceedings proposed to be taken by you in connection with the sale and issuance
of the Shares.

                  It is our opinion that, upon completion of the proceedings
being taken or contemplated by us, as your counsel, to be taken prior to the
issuance of the Shares being sold by the Company and upon completion of the
proceedings being taken in order to permit such transactions to be carried out
in accordance with the securities laws of the various states where required, the
Shares being sold by the Company, when issued and sold in the manner described
in the Registration Statement and in accordance with the resolutions adopted by
the Board of Directors of the Company, will be legally and validly issued, fully
paid and non-assessable.

                  We consent to the use of this opinion as an exhibit to said
Registration Statement and further consent to the use of our name wherever
appearing in said Registration Statement, including the prospectus constituting
a part thereof, and in any amendment or supplement thereto.

                                            Very truly yours,


                                            /s/ Gunderson Dettmer Stough
                                            Villeneuve Franklin & Hachigian, LLP