Exhibit 5.1


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                                                     December 5, 2003

Ascential Software Corporation
50 Washington Street
Westborough, MA  01581-1021

      Re:   Second Restated 1994 Stock Option and Award Plan
            Second Restated 1989 Outside Directors Stock Option Plan
            Second Restated 1997 Employee Stock Purchase Plan
            Amended and Restated 1998 Non-Statutory Stock Option and Award Plan
            1997 Non-Statutory Stock Option Plan

Ladies and Gentlemen:

      We have assisted in the preparation of a Registration Statement on Form
S-8 (the "Registration Statement") to be filed with the Securities and Exchange
Commission (the "Commission") under the Securities Act of 1933, as amended (the
"Securities Act"), relating to an aggregate of 6,050,000 shares of common stock,
$0.01 par value per share (the "Shares"), of Ascential Software Corporation, a
Delaware corporation (the "Company"), issuable under the Company's Second
Restated 1994 Stock Option and Award Plan, Second Restated 1989 Outside
Directors Stock Option Plan, Second Restated 1997 Employee Stock Purchase Plan,
Amended and Restated 1998 Non-Statutory Stock Option and Award Plan and 1997
Non-Statutory Stock Option Plan (collectively, the "Plans").

      We have examined the Restated Certificate of Incorporation and Second
Amended and Restated By-Laws of the Company, each as amended and restated to
date, and originals, or copies certified to our satisfaction, of all pertinent
records of the meetings of the directors and stockholders of the Company, the
Registration Statement and such other documents relating to the Company as we
have deemed material for the purposes of this opinion.

      In our examination of the foregoing documents, we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to us
as originals, the conformity to original documents of all documents submitted to
us as certified, photostatic or other copies, the authenticity of the originals
of any such documents and the legal competence of all signatories to such
documents.

      We assume that the appropriate action will be taken, prior to the offer
and sale of the Shares in accordance with the Plans, to register and qualify the
Shares for sale under all applicable state securities or "blue sky" laws.


BOSTON  LONDON  MUNICH  NEW YORK  OXFORD  PRINCETON  RESTON  WALTHAM  WASHINGTON
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       Hale and Dorr LLP is a Massachusetts Limited Liability Partnership

Ascential Software Corporation
December 5, 2003
Page 2


      We express no opinion herein as to the laws of any state or jurisdiction
other than the state laws of The Commonwealth of Massachusetts, the General
Corporation Law of the State of Delaware and the federal laws of the United
States of America.

      It is understood that this opinion is to be used only in connection with
the offer and sale of the Shares while the Registration Statement is in effect.

      Please note that we are opining only as to the matters expressly set forth
herein, and no opinion should be inferred as to any other matters.

      Based on the foregoing, we are of the opinion that the Shares have been
duly authorized for issuance and, when the Shares are issued and paid for in
accordance with the terms and conditions of the Plans, the Shares will be
validly issued, fully paid and nonassessable.

      We hereby consent to the filing of this opinion with the Commission in
connection with the Registration Statement in accordance with the requirements
of Item 601(b)(5) of Regulation S-K under the Securities Act. In giving such
consent, we do not hereby admit that we are in the category of persons whose
consent is required under Section 7 of the Securities Act or the rules and
regulations of the Commission.


                                                     Very truly yours,

                                                     /s/ Hale and Dorr LLP

                                                     HALE AND DORR LLP