As filed with the Securities and Exchange Commission on October 10, 1997


                                                       Registration No. 33-93048
================================================================================


                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                             --------------------
                       POST-EFFECTIVE AMENDMENT NO. 1 TO
                                    FORM S-8
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
                             --------------------
                                   ATMI, INC.
             (Exact name of registrant as specified in its charter)

          Delaware                                         06-1481060
(State or other jurisdiction of                          (I.R.S. Employer
incorporation or organization)                          Identification No.)

                             --------------------
                 7 Commerce Drive, Danbury, Connecticut  06810
             (Address of Principal Executive Offices)  (Zip Code)
                             --------------------
                      Advanced Technology Materials, Inc.
                                1995 Stock Plan
                            (Full title of the plan)
                             --------------------
                               Eugene G. Banucci
                                   President
                                   ATMI, Inc.
                                7 Commerce Drive
                           Danbury, Connecticut 06810
                    (Name and address of agent for service)
                             --------------------

                                 (203) 794-1100
         (Telephone number, including area code, of agent for service)
                             --------------------
                                        
                                    Copy to:

                              Frank J. Marco, Esq.
                             Shipman & Goodwin LLP
                                One American Row
                          Hartford, Connecticut 06103
                                 (203) 251-5000
================================================================================

 
                       INTRODUCTORY STATEMENT NOT FORMING
                               PART OF PROSPECTUS
                                        
    Pursuant to Rule 414 under the Securities Act of 1933, as amended (the
"Securities Act"), ATMI, Inc. (the "Company") hereby amends the Form S-8,
Registration No. 33-93048, filed by Advanced Technology Materials, Inc. ("ATMI")
on June 7, 1995, relating to shares of ATMI's common stock, par value $.01 per
share ("ATMI Common Stock"), issued or reserved for issuance pursuant to awards
granted under ATMI's 1995 Stock Plan (the "Plan").  The Plan was previously a
stock plan of ATMI, formerly the sole stockholder of the Company.  On October
10, 1997, pursuant to an Agreement and Plan of Merger and Exchange dated as of
April 7, 1997 (the "Merger and Exchange Agreement"), Alamo Merger, Inc., a
wholly-owned subsidiary of the Company, merged with and into ATMI, with ATMI as
the surviving corporation (the "Merger").  As a result of the Merger, ATMI
became a wholly-owned subsidiary of the Company.  Upon consummation of the
Merger, each outstanding share of ATMI Common Stock was converted into the right
to receive one share of the Company's common stock, par value $.01 per share
("Company Common Stock").  In addition, pursuant to the Merger and Exchange
Agreement, each outstanding option and warrant to purchase ATMI Common Stock,
including options issued under the Plan, entitle the holder thereof to purchase,
on identical terms, an equivalent number of shares of Company Common Stock.

    In accordance with Rule 414 under the Securities Act, the Company, as the
successor registrant to ATMI, hereby expressly adopts this registration
statement as its own for all purposes of the Securities Act and the Securities
Exchange Act of 1934, as amended.  The Plan shall continue to be known as the
Advanced Technology Materials, Inc. 1995 Stock Plan.  In addition, the Plan
shall continue to cover directors, employees or consultants, as the case may be,
of ATMI, as well as of the Company and its other subsidiaries.  However,
outstanding options to purchase shares of ATMI Common Stock shall entitle the
holder thereof to purchase an equivalent number of shares of Company Common
Stock.  Furthermore, the sponsor of the Plan shall be the Company rather than
ATMI.

    The registration fee was paid at the time of the original filing of the
registration statement.

                                      -1-

 
                                    PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT
                                        
Item 3.  Incorporation of Documents by Reference.

         In addition to the materials previously incorporated by reference into
this registration statement by ATMI, the Company hereby incorporates by
reference the Company's Current Report on Form 8-K dated October 10, 1997.

         All documents subsequently filed by the Company pursuant to Sections
13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to the filing of a post-
effective amendment which indicates that all securities offered hereby have been
sold or which deregisters all such securities then remaining unsold, shall be
deemed to be incorporated by reference in this registration statement and to be
part thereof from the date of filing of such documents.  Any statement contained
in a document incorporated or deemed to be incorporated by reference in this
registration statement shall be deemed to be modified or superseded for purposes
of this registration statement to the extent that a statement contained herein
or in any other subsequently filed document which also is or is deemed to be
incorporated by reference herein modifies or supersedes such statement.  Any
such statement so modified or superseded shall not be deemed, except as so
modified or superseded, to constitute a part of this registration statement.

Item 4.  Description of Securities.

         This Item is not applicable.

Item 5.  Interests of Named Experts and Counsel.

         As of the date of this post-effective amendment to this registration
statement, 7,714 shares of Company Common Stock are beneficially owned by
lawyers employed at Shipman & Goodwin LLP, counsel to the Company.

Item 6.  Indemnification of Directors and Officers.

         The Company's Certificate of Incorporation provides that the personal
liability of the directors of the Company shall be limited to the fullest extent
permitted by the provisions of Section 102(b)(7) of the Delaware General
Corporation Law, as amended (the "DGCL").  Section 102(b)(7) of the DGCL
generally provides that no director shall be liable personally to the Company or
its stockholders for monetary damages for breach of fiduciary duty as a
director; however, the Certificate of Incorporation does not eliminate the
liability of a director for (i) any breach of the director's duty of loyalty to
the Company or its stockholders; (ii) acts or omissions not in good faith or
that involve intentional misconduct or a knowing violation of law; (iii) acts or
omissions in respect of certain unlawful dividend payments or stock redemptions
or repurchases; or (iv) any transaction from which such director derives
improper personal benefit. The effect of this provision is to eliminate the

                                     II-1

 
rights of the Company and its stockholders (through stockholders' derivative
suits on behalf of the Company) to recover monetary damages against a director
for breach of his or her fiduciary duty of care as a director (including
breaches resulting from negligent or grossly negligent behavior) except in the
situations described in clauses (i) through (iv) above. The limitations
summarized above, however, do not affect the ability of the Company or its
stockholders to seek nonmonetary remedies, such as an injunction or rescission,
against a director for breach of his or her fiduciary duty.

      In addition, the Company's Certificate of Incorporation provides that the
Company shall, to the fullest extent permitted by Section 145 of the DGCL,
indemnify all persons whom it may indemnify pursuant to Section 145 of the DGCL.
Section 145 of the DGCL permits a company to indemnify an officer or director
who was or is a party or is threatened to be made a party to any proceeding
because of his or her position, if the officer or director acted in good faith
and in a manner he or she reasonably believed to be in or not opposed to the
best interests of the Company and, with respect to any criminal action or
proceeding, had no reasonable cause to believe his or her conduct was unlawful.
Insofar as indemnification for liabilities arising under the Securities Act may
be permitted to directors, officers, or persons controlling the Company pursuant
to the foregoing provisions, the Company has been informed that in the opinion
of the Securities and Exchange Commission, such indemnification is against
public policy as expressed in the Securities Act and is therefore unenforceable.

     The Company maintains insurance for officers and directors against certain
liabilities, including liabilities under the Securities Act.  The effect of this
insurance is to indemnify any officer or director of the Company against
expenses, including, without limitation, attorneys' fees, judgments, fines and
amounts paid in settlement, incurred by an officer or director upon a
determination that such person acted in good faith.

Item 7.  Exemption from Registration Claimed.

         This Item is not applicable.

                                     II-2

 
Item 8.  Exhibits.

 
 
Exhibit No.         Description
- ----------          -----------
          
   4.1(a)   Certificate of Incorporation of the Company (Exhibit 3.01 to the
            Company's Registration Statement on Form S-4, filed September 10,
            1997, File No. 333-35323 (the "Form S-4 Registration Statement")).
            (1)

   4.1(b)   Certificate of Amendment to Certificate of Incorporation. (2)

   4.2      Bylaws of the Company (Exhibit 3.02 to the Form S-4 Registration
            Statement). (1)

   5.1      Opinion of Shipman & Goodwin LLP as to the legality of the
            securities being registered. (2)

  23.1      Consent of Shipman & Goodwin LLP (included in Exhibit 5.1). (2)

  23.2      Consent of Ernst & Young LLP. (2)

  23.3      Consent of Price Waterhouse LLP. (2)

  24.1      Power of Attorney (included in the signature page of this
            registration statement). (2)

  99.1      ATMI's 1995 Stock Plan. (3)
 
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(1)  Incorporated by reference.

(2)  Filed herewith.

(3)  Previously filed.


Item 9.  Undertakings.

         (a) The undersigned registrant hereby undertakes:

             (1) To file, during any period in which offers or sales are being
made, a post-effective amendment to this registration statement to include any
material information with respect to the plan of distribution not previously
disclosed in the registration statement or any material change to such
information in the registration statement.

             (2) That, for the purpose of determining any liability under the
Securities Act, each such post-effective amendment shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

             (3) To remove from registration by means of post-effective
amendment any of the securities being registered which remain unsold at the
termination of the offering.

                                     II-3

 
      (b) The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
registrant's annual report pursuant to Section 13 or Section 15(d) of the
Exchange Act of 1934 (and, where applicable, each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Exchange Act of 1934) that
is incorporated by reference in the registration statement shall be deemed to be
a new registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

      (c) Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to directors, officers and controlling persons
of the registrant pursuant to the foregoing provisions, or otherwise, the
registrant has been advised that in the opinion of the Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Securities Act and is, therefore, unenforceable.  In the event that a claim for
indemnification against such liabilities (other than the payment by the
registrant of expenses incurred or paid by a director, officer or controlling
person of the registrant in the successful defense of any action, suit or
proceeding) is asserted by such director, officer or controlling person in
connection with the securities being registered, the registrant will, unless in
the opinion of its counsel the matter has been settled by controlling precedent,
submit to a court of appropriate jurisdiction the question  whether such
indemnification by it is against public policy as expressed in the Securities
Act and will be governed by the final adjudication of such issue.

                                     II-4

 
                                   SIGNATURES
                                        
     Pursuant to requirements of the Securities Act of 1933, the registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-8 and has duly caused this registration
statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Danbury, State of Connecticut, on October 10, 1997.

                                        ATMI,  INC.


                                        By: /s/ Eugene G. Banucci
                                           -----------------------------
                                           Eugene G. Banucci,
                                           President

                               POWER OF ATTORNEY
                                        
     Know All Persons by These Presents, that each person whose signature
appears below constitutes and appoints Eugene G. Banucci, Ph.D. and Daniel P.
Sharkey, and each of them, his true and lawful attorneys-in-fact and agents,
with full power of substitution and resubstitution for him and in his name,
place and stead, in any and all capacities to sign any and all amendments
(including post-effective amendments) to this Registration Statement, and to
file same, with all exhibits thereto, and other documents in connection
therewith, with full power and authority to do and perform each and every act
and thing requisite or necessary to be done in and about the premises, as fully
to all intents and purposes as he might or could do in person, hereby ratifying
and confirming all that said attorneys-in-fact and agents or any of them, of
their or his substitute or substitutes, may lawfully do or cause to be done by
virtue hereof.

                              -------------------

     Pursuant to the requirements of the Securities Act of 1933, this
registration statement has been signed by the following persons in the
capacities and on the dates indicated.



                   Signature                                     Title                         Date
                   ---------                                     -----                         ----          
                                                                                   
             /s/ Eugene G. Banucci               President and Director (principal       October 10, 1997  
- -----------------------------------------------  executive officer)
          Eugene G. Banucci, Ph.D.
 
             /s/ Daniel P. Sharkey               Treasurer and Secretary (principal      October 10, 1997  
- -----------------------------------------------  financial and accounting officer)
                Daniel P. Sharkey
 
               /s/ Mark A. Adley                 Director                                October 10, 1997  
- -----------------------------------------------
                  Mark A. Adley
 
             /s/ John A. Armstrong               Director                                October 10, 1997  
- -----------------------------------------------
                John A. Armstrong
 
               /s/ Robert S. Hillas              Director                                October 10, 1997  
- -----------------------------------------------
                  Robert S. Hillas
 
               /s/ Stephen H. Mahle              Director                                October 10, 1997  
- -----------------------------------------------
                  Stephen H. Mahle


                                     II-5

 
                                 Exhibit Index
                                 -------------
 
 
                                                                            Sequentially
Exhibit No.     Description                                                 Numbered Page
- --------------  -----------                                                 -------------
                                                                       
    4.1(a)      Certificate of Incorporation of the Company (Exhibit
                3.01 to the Company's Registration Statement on Form
                S-4, filed September 10, 1997, File No. 333-35323 (the
                "Form S-4 Registration Statement"). (1)

    4.1(b)      Certificate of Amendment to Certificate of Incorporation.
                (2)

      4.2       Bylaws of the Company (Exhibit 3.02 to the Form S-4
                Registration Statement). (1)

      5.1       Opinion of Shipman & Goodwin LLP as to the legality of
                the securities being registered. (2)

     23.1       Consent of Shipman & Goodwin LLP (included in Exhibit
                5.1). (2)

     23.2       Consent of Ernst & Young LLP. (2)

     23.3       Consent of Price Waterhouse LLP. (2)

     24.1       Power of Attorney (included in the signature page of
                this registration statement). (2)

     99.1       ATMI 's 1995 Stock Plan. (3)

- --------------------------
 
(1) Incorporated by reference.

(2) Filed herewith.

(3) Previously filed.