Exhibit 5.1


                              October 10, 1997


Board of Directors
ATMI, Inc.
7 Commerce Drive
Danbury, CT  06810

     Re:  Post-Effective Amendment No. 1 to Registration
          Statement on Form S-8 (Registration No. 33-77060) Relating
          to Shares of Common Stock of ATMI, Inc. Issuable Under
          the Advanced Technology Materials, Inc. 1987 Stock Plan
          -------------------------------------------------------

Gentlemen:

     As counsel for ATMI, Inc., a Delaware corporation (the "Company"), we are
furnishing you with this opinion in connection with the issuance of a maximum of
643,291 shares of Common Stock of the Company (the "Shares") pursuant to the
above-referenced Plan (the "Plan"), to which the above-referenced Registration
Statement relates.

     We have examined such originals or copies of corporate records of the
Company, certificates of public officials and of officers of the Company and
other documents, have obtained such assurances from officers and representatives
of the Company, have made such other factual inquiries, and have made such
examination of law, as we have deemed proper and necessary to enable us to
render this opinion.

     Based on the foregoing, it is our opinion that the Shares will, when issued
as contemplated by the Plan and said Registration Statement, be duly authorized
and legally issued, fully paid and non-assessable.

     We hereby consent to the use of this opinion as an exhibit to the
Registration Statement.

                                       Very truly yours,

                                       /s/ Shipman & Goodwin LLP