SCHEDULE 14A

                                 (RULE 14a-101)

                    INFORMATION REQUIRED IN PROXY STATEMENT

                            SCHEDULE 14A INFORMATION

                PROXY STATEMENT PURSUANT TO SECTION 14(a) OF THE

                        SECURITIES EXCHANGE ACT OF 1934

Filed by the Registrant [X]

Filed by a Party other than the Registrant [ ]

Check the appropriate box:

[ ]  Preliminary Proxy Statement

[ ]  Confidential, for Use of the Commission Only (as permitted by Rule
     14a-6(e)(2))

[X]  Definitive Proxy Statement

[ ]  Definitive Additional Materials

[ ]  Soliciting Material Under Rule 14a-12

                                   ATMI, INC.
- --------------------------------------------------------------------------------
                (Name of Registrant as Specified in Its Charter)

- --------------------------------------------------------------------------------
    (Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

[X]  No fee required

[ ]  Fee computed on table below per Exchange Act Rules 14a-6(i)(1) and 0-11

     (1)  Title of each class of securities to which transaction applies:

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

     (2)  Aggregate number of securities to which transaction applies:

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

     (3)  Per unit price or other underlying value of transaction computed
          pursuant to Exchange Act Rule 0-11 (set forth the amount on which the
          filing fee is calculated and state how it was determined):

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

     (4)  Proposed maximum aggregate value of transaction:

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

     (5)  Total fee paid:

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

[ ]  Fee paid previously with preliminary materials.

[ ]  Check box if any part of the fee is offset as provided by Exchange Act Rule
     0-11(a)(2) and identify the filing for which the offsetting fee was paid
     previously. Identify the previous filing by registration statement number,
     or the Form or Schedule and the date of its filing.

     (1)  Amount Previously Paid:

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

     (2)  Form, Schedule or Registration Statement No.:

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

     (3)  Filing Party:

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

     (4)  Date Filed:

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


                                  (ATMI LOGO)

                                   ATMI, INC.
                                7 COMMERCE DRIVE
                           DANBURY, CONNECTICUT 06810
                                 (203) 794-1100

                                                                  April 21, 2004

Dear Stockholder:

     You are cordially invited to attend the annual meeting of stockholders of
ATMI, Inc., which will be held at our corporate offices located at 6 Commerce
Drive, Danbury, Connecticut 06810 at 10:00 a.m. on Tuesday, May 25, 2004. On the
following pages, you will find the formal Notice of Annual Meeting and Proxy
Statement.

     Whether or not you plan to attend the annual meeting in person, it is
important that your shares are represented and voted at the annual meeting.
Accordingly, please date, sign, and return the enclosed proxy card promptly.

     I hope that you will attend the meeting. I look forward to seeing you
there.

                                          Sincerely,

                                          /s/ Eugene G. Banucci
                                          --------------------------------------
                                          EUGENE G. BANUCCI
                                          Chief Executive Officer and Chairman
                                          of the Board


                                  (ATMI LOGO)

                    NOTICE OF ANNUAL MEETING OF STOCKHOLDERS
                            TO BE HELD MAY 25, 2004

To Our Stockholders:

     The 2004 annual meeting of stockholders of ATMI, Inc. (the "Company") will
be held at the Company's corporate offices located at 6 Commerce Drive, Danbury,
Connecticut on Tuesday, May 25, 2004 at 10:00 a.m. (local time) for the
following purposes:

1.  To elect two Class I directors for a term expiring at the annual meeting of
    stockholders in 2007;

2.  To ratify the appointment by the Board of Directors of Ernst & Young LLP as
    the Company's independent auditors for the fiscal year ending December 31,
    2004; and

3.  To transact such other business as may properly come before the meeting or
    any adjournment or postponement thereof.

     Only holders of record of common stock (NNM: ATMI) at the close of business
on April 12, 2004 are entitled to receive notice of, and to vote at, the meeting
and any adjournments or postponements of the meeting.

                                          By order of the Board of Directors,

                                          /s/ Daniel P. Sharkey
                                          --------------------------------------
                                          Daniel P. Sharkey
                                          Secretary

Dated: April 21, 2004
Danbury, Connecticut

WHETHER OR NOT YOU EXPECT TO ATTEND THE ANNUAL MEETING, PLEASE MARK, SIGN, DATE,
AND RETURN THE ENCLOSED PROXY PROMPTLY IN THE ENCLOSED POSTAGE-PREPAID ENVELOPE.
ANY PERSON GIVING A PROXY HAS THE POWER TO REVOKE IT AT ANY TIME BEFORE ITS
EXERCISE AND, IF PRESENT AT THE MEETING, MAY WITHDRAW IT AND VOTE IN PERSON.


                                  [ATMI LOGO]

                                   ATMI, INC.
                                7 COMMERCE DRIVE
                           DANBURY, CONNECTICUT 06810

                                PROXY STATEMENT
                         ANNUAL MEETING OF STOCKHOLDERS
                                  MAY 25, 2004

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

     This proxy statement is being furnished to the holders of common stock (the
"Common Stock") of ATMI, Inc. (the "Company") in connection with the
solicitation of proxies by the Board of Directors of the Company for use at the
Company's annual meeting of stockholders (the "Annual Meeting"), to be held on
May 25, 2004 at the Company's corporate offices located at 6 Commerce Drive,
Danbury, Connecticut, and at any adjournments or postponements thereof.

     This proxy statement, the foregoing Notice of Annual Meeting, the enclosed
form of proxy, and the Company's 2003 Annual Report to Stockholders are first
being mailed or given to stockholders on or about April 21, 2004. As used in
this proxy statement, references to the "Company" include references to ATMI,
Inc. and to its predecessor registrant, Advanced Technology Materials, Inc.

WHAT IS THE PURPOSE OF THE ANNUAL MEETING?

     At our annual meeting, stockholders will act upon the matters outlined in
the Notice of Annual Meeting, including the election of Class I directors, the
ratification of the Company's independent auditors and any other matters that
may properly come before the Annual Meeting. In addition, management will report
on the performance of the Company and respond to questions from stockholders.

WHO IS ENTITLED TO VOTE AT THE ANNUAL MEETING?

     Only holders of record of Common Stock at the close of business on April
12, 2004, the record date for the Annual Meeting (the "Record Date"), are
entitled to receive notice of and to participate in the Annual Meeting. If you
were a holder of record of Common Stock on that date, you will be entitled to
vote all of the shares that you held on that date at the Annual Meeting, or any
postponements or adjournments of the Annual Meeting.

WHAT ARE THE VOTING RIGHTS OF THE HOLDERS OF COMMON STOCK?

     Each outstanding share of Common Stock will be entitled to one vote on each
matter considered at the Annual Meeting.

WHO CAN ATTEND THE ANNUAL MEETING?

     Subject to space availability, all stockholders as of the Record Date, or
their duly appointed proxies, may attend the Annual Meeting. Since seating is
limited, admission to the Annual Meeting will be on a first-come, first-served
basis.

     Please also note that if you hold your shares in "street name" (that is,
through a broker or other nominee), you will need to bring a copy of a brokerage
statement reflecting your stock ownership as of the Record Date and check in at
the registration desk at the Annual Meeting.


WHAT CONSTITUTES A QUORUM?

     The presence at the Annual Meeting, in person or by proxy, of the holders
of a majority of the aggregate voting power of the Common Stock outstanding on
the Record Date will constitute a quorum, permitting the Annual Meeting to
conduct its business. As of the Record Date, 31,157,285 shares of Common Stock,
representing the same number of votes, were outstanding. Thus, the presence of
the holders of Common Stock representing at least 15,578,643 votes will be
required to establish a quorum.

     Proxies received but marked as abstentions and broker non-votes will be
included in the calculation of the number of votes considered to be present at
the Annual Meeting for purposes of determining whether a quorum is present.

HOW DO I VOTE?

     If you complete and properly sign the accompanying proxy card and return it
to the Company, your shares of Common Stock will be voted as directed on the
proxy card. If you are a registered stockholder and attend the Annual Meeting,
you may deliver your completed proxy card in person. "Street name" stockholders
who wish to vote at the Annual Meeting will need to obtain a proxy form from the
institution that holds their shares.

CAN I CHANGE MY VOTE AFTER I RETURN MY PROXY CARD?

     Yes. Even after you have submitted your proxy, you may revoke or change
your vote at any time before the proxy is exercised by filing with the Corporate
Secretary of the Company either a notice of revocation or a duly executed proxy
bearing a later date. The powers of the proxy holders will be suspended if you
attend the Annual Meeting in person and so request, although attendance at the
Annual Meeting will not by itself revoke a previously granted proxy.

WHAT ARE THE BOARD'S RECOMMENDATIONS?

     Unless you give other instructions on your proxy card, the persons named as
proxy holders on the proxy card will vote in accordance with the recommendations
of the Board. The Board's recommendation is set forth together with the
description of each item in this proxy statement. In summary, the Board
recommends a vote:

     - FOR election of the nominated slate of Class I directors (see Proposal
       No. 1); and

     - FOR ratification of the appointment of Ernst & Young LLP as the Company's
       independent auditors for fiscal year ending December 31, 2004 (see
       Proposal No. 2).

     With respect to any other matter that properly comes before the Annual
Meeting, the proxy holders will vote as recommended by the Board or, if no
recommendation is given, in their own discretion.

WHAT VOTE IS REQUIRED TO APPROVE EACH ITEM?

     Election of Class I Directors.  The affirmative vote of a plurality of the
votes cast at the Annual Meeting is required for the election of the Class I
directors. A properly executed proxy marked "Withhold authority" with respect to
the election of one or more Class I directors will not be voted with respect to
the nominee for Class I director or directors indicated, although it will be
counted for purposes of determining whether there is a quorum.

     Ratification of Appointment of Ernst & Young LLP.  The affirmative vote of
the holders of a majority of the shares represented in person or by proxy and
entitled to vote at the Annual Meeting will be required for approval. A properly
executed proxy marked "Abstain" with respect to any such matter will not be
voted, although it will be counted for purposes of determining whether there is
a quorum. Accordingly, an abstention will have the effect of a negative vote.

     Under the rules that govern brokers who have record ownership of shares
that are held in "street name" for their clients, who are the beneficial owners
of the shares, brokers have discretion to vote these shares on

                                        2


routine matters but not on non-routine matters. Your broker will have
discretionary authority to vote your shares on each of the proposals to be
considered at the Annual Meeting, which are both routine matters. Thus, if you
do not otherwise instruct your broker, the broker may turn in a proxy card
voting your shares "FOR" Proposal No. 1 and "FOR" Proposal No. 2. A "broker
non-vote" occurs when a broker expressly indicates on a proxy card that it is
not voting on a matter. To the extent your broker submits a broker non-vote with
respect to your shares on a proposal, your shares will not be deemed "votes
cast" with respect to that proposal and will have the same effect as an
abstention with respect to that proposal. Accordingly, broker non-votes will
have no effect on the outcome of the vote with respect to the election of the
Class I directors and will have the same effect as a vote against the proposal
to ratify the appointment of Ernst & Young LLP as the Company's independent
auditors.

                                STOCK OWNERSHIP

     The following table sets forth certain information known to the Company
regarding the beneficial ownership of Common Stock as of the Record Date, by:
(i) each executive officer of the Company named in the Summary Compensation
Table on page 8; (ii) each director and nominee for director of the Company;
(iii) each person known by the Company to own beneficially more than five
percent (5%) of the outstanding Common Stock of the Company; and (iv) all
current directors and executive officers of the Company as a group. Except as
indicated by footnote, all shares are owned directly. Except as indicated by
footnote, and subject to community property laws where applicable, the persons
named in the table have sole voting and investment power with respect to all
shares of Common Stock shown as beneficially owned by them.

<Table>
<Caption>
                                                                 SHARES
                                                              BENEFICIALLY    PERCENT
NAME AND ADDRESS OF BENEFICIAL OWNER(1)                          OWNED        OF CLASS
- ---------------------------------------                       ------------    --------
                                                                        
FMR Corp.(2)................................................   4,658,019       14.95%
  82 Devonshire Street
  Boston, Massachusetts 02109
T. Rowe Price Associates, Inc.(3)...........................   3,874,200       12.43%
  100 E. Pratt Street
  Baltimore, Maryland 21202
Wellington Management Company, LLP(4).......................   1,722,940        5.53%
  75 State Street
  Boston, Massachusetts 02109
Eugene G. Banucci(5)........................................     502,148         1.6%
Daniel P. Sharkey(6)........................................     184,052        *
Douglas A. Neugold(7).......................................     178,919        *
Robert S. Hillas(8).........................................      91,459        *
Mark A. Adley(9)............................................      85,995        *
Stephen H. Mahle(10)........................................      59,545        *
C. Douglas Marsh(11)........................................      63,194        *
Michael J. Yomazzo(12)......................................      56,566        *
All current directors and executive officers as a group (8
  persons)(13)..............................................   1,221,878         3.9%
</Table>

- ---------------
  * Represents less than 1% of the outstanding Common Stock.

 (1) Except as otherwise noted, the address for all stockholders is c/o ATMI,
     Inc., 7 Commerce Drive, Danbury, Connecticut 06810.

 (2) As reported on Schedule 13G/A, as filed with the Securities and Exchange
     Commission (the "SEC") on February 17, 2004.

 (3) As reported on Schedule 13G/A, as filed with the SEC on February 13, 2004.

                                        3


 (4)  As reported on Schedule 13G, as filed with the SEC on February 13, 2004.

 (5)  Includes 287,000 shares issuable upon exercise of options that are
      exercisable within 60 days of the Record Date, 10,884 restricted shares
      issued as of January 2, 2004, and 19,021 shares either owned or issuable
      upon exercise of options within 60 days of the Record Date by Dr.
      Banucci's spouse. Dr. Banucci disclaims beneficial ownership of the shares
      held by his spouse.

 (6)  Includes 143,000 shares issuable upon exercise of options that are
      exercisable within 60 days of the Record Date and 5,442 restricted shares
      issued as of January 2, 2004.

 (7)  Includes 170,000 shares issuable upon exercise of options that are
      exercisable within 60 days of the Record Date and 8,163 restricted shares
      issued as of January 2, 2004.

 (8)  Includes 55,000 shares issuable upon exercise of options that are
      exercisable within 60 days of the Record Date, 3,481 phantom stock units
      (as described below under the heading "Director Compensation"), which are
      automatically convertible into shares of common stock upon the occurrence
      of certain specified events, including death or termination of status as a
      director ("Phantom Stock Units"), 1,701 restricted shares issued as of
      January 2, 2004, and 10,000 shares owned by the Hillas Family Limited
      Partnership.

 (9)  Includes 74,917 shares issuable upon exercise of options that are
      exercisable within 60 days of the Record Date, 3,377 Phantom Stock Units,
      and 1,701 restricted shares issued as of January 2, 2004

(10)  Includes 54,500 shares issuable upon exercise of options that are
      exercisable within 60 days of the Record Date, 2,744 Phantom Stock Units,
      and 1,701 restricted shares issued as of January 2, 2004.

(11)  Includes 50,000 shares issuable upon exercise of options that are
      exercisable within 60 days of the Record Date, 2,835 Phantom Stock Units,
      1,701 restricted shares issued as of January 2, 2004 and 8,658 shares in a
      trust which Mr. Marsh, or a member of his immediate family, is a
      beneficiary.

(12)  Includes 50,000 shares issuable upon exercise of options that are
      exercisable within 60 days of the Record Date, 3,365 Phantom Stock Units,
      1,701 restricted shares issued as of January 2, 2004 and 1,500 shares in a
      trust which Mr. Yomazzo, or a member of his immediate family, is a
      beneficiary.

(13)  Includes 893,417 shares issuable to executive officers, directors, and
      their spouses pursuant to options that are exercisable within 60 days of
      the Record Date, 15,803 Phantom Stock Units, and 32,994 restricted shares
      issued as of January 2, 2004.

SECTION 16(a) BENEFICIAL OWNERSHIP REPORTING COMPLIANCE

     Section 16(a) of the Securities Exchange Act of 1934, as amended (the
"Exchange Act"), requires the Company's executive officers and directors, and
persons who beneficially own more than ten percent (10%) of the Common Stock, to
file reports of ownership and changes in ownership with the SEC, and to furnish
the Company with copies of all such forms they file. Based solely on its review
of filings with the SEC, copies of such filings received by the Company, or
written representations from certain reporting persons, the Company believes
that the Company's executive officers and directors, and persons who
beneficially own more than ten percent (10%) of the Common Stock complied with
Section 16(a) of the Exchange Act during the fiscal year ended December 31,
2003.

                                 PROPOSAL NO. 1

                             ELECTION OF DIRECTORS

     The Board of Directors is classified into three classes. The two directors
serving in Class I have terms expiring at this Annual Meeting. The Board of
Directors has nominated the Class I directors currently serving on the Board of
Directors, Robert S. Hillas and Michael J. Yomazzo, for election to serve as
Class I directors of the Company for a three-year term expiring at the Company's
annual meeting of stockholders in 2007, and until their successors are duly
elected and qualified, or until their earlier resignation, death, or removal.
Each of the nominees has indicated a willingness to serve as a director, but if
for any reason any nominee should be unavailable to serve as a director at the
time of the Annual Meeting, a contingency which the Board of

                                        4


Directors does not expect, a different person designated by the Board of
Directors may be nominated in his stead.

CLASS I DIRECTOR NOMINEES FOR TERMS EXPIRING IN 2007

     The following table sets forth information regarding the nominees for
re-election as Class I Directors:

<Table>
<Caption>
NAME                                                          AGE   POSITION
- ----                                                          ---   --------
                                                              
Robert S. Hillas............................................  55    Director
Michael J. Yomazzo..........................................  61    Director
</Table>

     ROBERT S. HILLAS has served as a director of the Company since 1987. From
April 1998 to March 2003, Mr. Hillas was the President, Chief Executive Officer,
and Chairman of the Board of Envirogen, Inc., an environmental systems and
services company. From 1993 to April 1998, Mr. Hillas served as a Managing
Director of E.M. Warburg, Pincus & Co. LLC, a venture banking and asset
management firm.

     MICHAEL J. YOMAZZO has served as a director of the Company since April
2000. From January 1, 1999 to January 2004, Mr. Yomazzo was the Vice Chairman of
the Board of Directors of Photronics, Inc., a manufacturer of photomasks, which
are photographic quartz plates containing microscopic images of electronic
circuits which are used in the manufacture of semiconductors, and had been a
member of that Board since 1977. Mr. Yomazzo served as Chief Executive Officer
of Photronics from August 1997 until December 1998 and as President from January
1994 until December 1998.

CONTINUING DIRECTORS

     The following table holds information regarding directors whose terms
continue after the Annual Meeting. The terms for Directors in Class II expire at
the 2005 Annual Meeting of Stockholders of the Company, and the terms for
Directors in Class III expire at the 2006 Annual Meeting of Stockholders of the
Company.

<Table>
<Caption>
NAME                                 AGE   CLASS   POSITION
- ----                                 ---   -----   --------
                                          
Mark A. Adley......................  44      II    Director
Eugene G. Banucci..................  60      II    Chief Executive Officer, Chairman
                                                   of the Board and Director
Stephen H. Mahle...................  58     III    Director
C. Douglas Marsh...................  58     III    Director
Douglas A. Neugold.................  45     III    President, Chief Operating Officer
</Table>

CLASS II DIRECTORS -- TERMS EXPIRING IN 2005

     MARK A. ADLEY has served as a director of the Company since 1991. Since
March 2002, Mr. Adley has been a Managing Director of Mergers & Acquisitions at
Banc of America Securities. From 1996 to 2001, Mr. Adley was a Managing Director
at Credit Suisse First Boston Corporation, an investment banking firm, where he
was a Director from 1994 to 1996.

     EUGENE G. BANUCCI, PH.D., a founder of the Company, has served as Chief
Executive Officer, Chairman of the Board and Director since 1986. Previously,
Dr. Banucci served in a variety of executive and managerial positions, including
serving as President from 1986 to April 2000. From 1984 to 1986, Dr. Banucci was
a director of American Cyanamid Company's Chemical Research Division, with
responsibility for the research, development, and technical service activities
of the Chemicals Group.

CLASS III DIRECTORS -- TERMS EXPIRING IN 2006

     STEPHEN H. MAHLE has served as a director of the Company since 1996. Since
January 1998, Mr. Mahle has been Senior Vice President of Medtronic, Inc., a
medical device manufacturer, and President of its cardiac rhythm management
business. From 1995 to 1997, Mr. Mahle was President of the Brady Pacing
Business, a

                                        5


division of Medtronic, Inc. From 1989 to 1995, Mr. Mahle served as Vice
President and General Manager of the Brady Pacing Business.

     C. DOUGLAS MARSH has served as a director of the Company since April 2000.
From July 1998 to April 2004, Mr. Marsh was the Vice President, Business
Integration, U.S. Investor Relations of ASM Lithography Holding NV, a seller of
photolithography equipment to the semiconductor industry. From 1991 to July
1998, Mr. Marsh served as Vice President, Worldwide Sales and President, U.S.
Operations at ASM Lithography.

     DOUGLAS A. NEUGOLD has served as President since May 2000 and as a director
since August 2003. Until his appointment as President, Mr. Neugold served as
Executive Vice President of the ATMI Materials segment since February 1999. In
January 1998, Mr. Neugold joined ATMI as Vice President of the NovaSource
division, and since July 1998, served as President of that division. Previously,
Mr. Neugold served in a variety of executive and managerial positions with the
Electronic Materials Division of Johnson Matthey. From 1995 to 1997, he served
as Vice President, and later as President of the Semiconductor Packages
business. From 1993 to 1995, Mr. Neugold served as Director of Asian Operations,
and before that served in a variety of business and marketing management
positions focused on semiconductor technology.

CORPORATE GOVERNANCE

WHO ARE THE CURRENT MEMBERS OF THE BOARD AND EACH COMMITTEE OF THE BOARD?

<Table>
<Caption>
                                                                           NOMINATING AND
                                               AUDIT     COMPENSATION   CORPORATE GOVERNANCE
DIRECTOR                                     COMMITTEE    COMMITTEE          COMMITTEE
- --------                                     ---------   ------------   --------------------
                                                               
Mark A. Adley..............................      *             *
Eugene G. Banucci..........................
Robert S. Hillas...........................     **                                *
Stephen H. Mahle...........................                   **
C. Douglas Marsh...........................                    *                  *
Douglas A. Neugold.........................
Michael J. Yomazzo.........................      *                               **
</Table>

- ---------------
  * Member

 ** Chairman

WHAT IS THE ROLE OF EACH COMMITTEE?

     The Board of Directors has a standing Audit Committee established in
accordance with section 3(a)(58)(A) of the Exchange Act, Compensation Committee
and Nominating and Corporate Governance Committee.

     Audit Committee.  The functions and responsibilities of the Audit Committee
are described in the written charter attached as Appendix A hereto and are
described in more detail below under the heading "Report of the Audit
Committee."  All of the members of the Audit Committee are independent within
the meaning of SEC regulations and the listing standards of the National
Association of Securities Dealers. In addition, the Board has determined that
each member of the Audit Committee is financially literate and that each of Mark
A. Adley, Robert S. Hillas and Michael J. Yomazzo is considered an "audit
committee financial expert" as defined in Item 401(h)(2) of Regulation S-K as
promulgated by the SEC and "independent" as defined in Item 7(d)(3)(iv) of
Schedule 14A under the Exchange Act. The Audit Committee met three times in
person during 2003, in addition to holding eight telephonic meetings to review
the Company's quarterly results.

     Compensation Committee.  The Compensation Committee is responsible for
reviewing the Company's compensation policies and practices and compensation of
senior officers of the Company. The Compensation Committee is also responsible
for administering the Company's 1987, 1995, 1997, 1998, 2000, and 2003 Stock

                                        6


Plans, including approving the grant of stock options thereunder. The
Compensation Committee met three times in person and twice by telephone during
2003.

     Nominating & Corporate Governance Committee.  The Nominating & Corporate
Governance Committee is responsible for (1) identifying individuals who are
qualified to become Board members, (2) selecting, or recommending that the Board
select, the composition of the committees of the Board, (3) monitoring a process
to assess Board effectiveness and (4) developing and recommending to the Board
the Company's corporate governance guidelines and principles. The Nominating &
Corporate Governance Committee does not presently operate under a written
charter; however, the members of the committee are in the process of developing
a charter and expect to adopt one in the near future. All of the members of the
Nominating & Corporate Governance Committee are independent within in the
meaning of the listing standards of the National Association of Securities
Dealers. The Nominating & Corporate Governance Committee met five times during
2003.

WHO IS THE BOARD'S PRESIDING INDEPENDENT DIRECTOR?

     During 2003, Michael J. Yomazzo was appointed as the lead independent
director of the Board of Directors. In this role, Mr. Yomazzo acts as the
chairman when the independent members of the Board of Directors are meeting in
executive sessions, and leads the deliberations of the Board's independent
directors on topics such as corporate governance and independence.

IS THERE A PROCESS IN PLACE FOR NOMINATING DIRECTORS?

     Historically, the Company has not been presented with a nominee for
director by any of its stockholders and accordingly, the Nominating & Corporate
Governance Committee does not have a formal process in place for nominating
directors but is actively in the process of developing one and expects to adopt
such a process in the near future. Nevertheless, the Nominating & Corporate
Governance Committee would consider qualified nominees recommended by the
stockholders in accordance with the procedures set forth in the Company's
Bylaws. Absent special circumstances, the Nominating & Corporate Governance
Committee will continue to nominate qualified incumbent directors whom the
Nominating & Corporate Governance Committee believes will continue to make an
important contribution to the Board. The Nominating & Corporate Governance
Committee generally will require that nominees be persons of sound ethical
character, be able to represent all stockholders fairly, have no material
conflicts of interest, have demonstrated professional achievement, have
meaningful experience and have a general appreciation of the major business
issues facing the Company.

HOW CAN STOCKHOLDERS COMMUNICATE WITH THE BOARD?

     Stockholders may contact any of the Company's directors (including the
Presiding Independent Director) by writing to them at ATMI, Inc., 6 Commerce
Drive, Danbury, Connecticut 06810. Historically, the Board has not received
stockholder communications and therefore, the Board does not have a formal
process in place for handling such communications; however, the Board is
actively evaluating such a process and expects to adopt one in the near future.

DO MEMBERS OF THE BOARD ATTEND THE ANNUAL MEETING?

     It has been the longstanding practice of the Company for all directors to
attend the Annual Meeting. All directors who were elected to the Board at the
last Annual Meeting were in attendance.

HOW OFTEN DID THE BOARD MEET DURING FISCAL 2003?

     The Board of Directors held six meetings in person during 2003, in addition
to three telephonic meetings during 2003. Each director attended at least 75% of
the aggregate of (i) the total number of meetings of the Board of Directors, and
(ii) the total number of meetings held by all committees of the Board on which
such director served.

                                        7


HOW ARE DIRECTORS COMPENSATED?

     Non-Employee Directors' Deferred Compensation Program.  The Company's
outside directors do not receive any cash compensation for serving on the Board
of Directors or any committee thereof, but do participate in the Company's
Non-Employee Directors' Deferred Compensation Program (the "Program"). Under the
Program, established in 2001, each outside director defers receipt of all
retainers and meeting fees on a quarterly basis into individual "phantom stock
accounts," established at the point of deferral, equivalent to phantom shares of
the Company's Common Stock valued at each quarter-end closing price. Each
outside Director receives $1,250 worth of phantom stock units per quarter as a
retainer, phantom stock units worth $1,000 per Board meeting, and $500 per
committee meeting attended. The phantom shares are exchangeable into shares of
Common Stock upon the occurrence of certain specified events, including the
death of the Director, or termination of his status as a Director. The outside
Directors are reimbursed for expenses incurred in connection with attending
meetings of the Board of Directors and any committee thereof.

     Options.  The Company has also made grants of stock options to its outside
directors as follows:

<Table>
<Caption>
DIRECTOR                                 DATE OF GRANT   NUMBER OF OPTIONS   EXERCISE PRICE
- --------                                 -------------   -----------------   --------------
                                                                    
Mark A. Adley..........................  May 1995             22,500             $ 8.50
                                         January 1998         25,500             $24.25
                                         April 2000            5,000             $40.13
                                         May 2000             25,000             $37.63
                                         May 2003              2,917             $19.83
Robert S. Hillas.......................  December 1994        22,500             $ 5.50
                                         January 1998         25,000             $24.25
                                         May 2000             25,000             $37.63
                                         January 2002          5,000             $23.85
                                         January 2003          5,000             $18.52
Stephen H. Mahle.......................  March 1996           22,500             $10.50
                                         January 1998          2,000             $24.25
                                         May 2000             25,000             $37.63
                                         January 2002          5,000             $23.85
                                         January 2003          5,000             $18.52
C. Douglas Marsh.......................  April 2000           50,000             $45.63
                                         January 2002          5,000             $23.85
                                         January 2003          5,000             $18.52
Michael J. Yomazzo.....................  April 2000           50,000             $45.63
                                         January 2002          5,000             $23.85
                                         January 2003          5,000             $18.52
</Table>

     In each case, the options were granted in consideration of future services
on the Board of Directors and the exercise price was equal to the fair market
value of the Common Stock on the date of grant. All options granted were subject
to certain vesting provisions.

                                        8


                                   MANAGEMENT

EXECUTIVE OFFICERS

     The following table sets forth certain information with respect to the
Company's executive officers (other than Eugene G. Banucci, for whom information
is set forth under Class II Directors and Douglas A. Neugold, for whom
information is set forth under Class III Directors, above):

<Table>
<Caption>
NAME                                        AGE   POSITION
- ----                                        ---   --------
                                            
Daniel P. Sharkey.........................  47    Vice President, Chief Financial Officer,
                                                  Treasurer, and Secretary
</Table>

     DANIEL P. SHARKEY has served as Chief Financial Officer since joining ATMI
in 1990, as Vice President and Treasurer since 1993, and as Secretary since
2004. From 1987 to 1990, Mr. Sharkey was Vice President of Finance and
Administration for Adage, Inc., a manufacturer of high-performance computer
graphics terminals. From 1983 to 1987, Mr. Sharkey was Corporate Controller for
CGX Corporation. Prior thereto, Mr. Sharkey served as an Audit Supervisor for
KPMG.

EXECUTIVE COMPENSATION

     The following table sets forth certain information regarding the
compensation paid by the Company for the years ended December 31, 2003, 2002,
and 2001 to the Company's Chief Executive Officer and both of the Company's
other executive officers (together, the "Named Executive Officers") for services
in all capacities to the Company and its subsidiaries.

                           SUMMARY COMPENSATION TABLE

<Table>
<Caption>
                                                                      LONG TERM
                                                                     COMPENSATION
                                                                        AWARDS
                                                      ANNUAL         ------------
                                                   COMPENSATION       SECURITIES
                                        FISCAL   -----------------    UNDERLYING        ALL OTHER
NAME & PRINCIPAL POSITION                YEAR    SALARY($)   BONUS    OPTIONS(#)    COMPENSATION($)(1)
- -------------------------               ------   ---------   -----   ------------   ------------------
                                                                     
Eugene G. Banucci.....................   2003     410,558      --       60,000            10,472
  Chief Executive Officer, Chairman      2002     373,731      --       50,000             8,610
  of the Board and Director              2001     375,962      --       75,000             8,520
Douglas A. Neugold....................   2003     310,693      --       60,000             7,500
  President and Chief Operating          2002     279,769      --       40,000             6,750
  Officer                                2001     290,577      --       55,000             6,750
Daniel P. Sharkey.....................   2003     225,962      --       30,000             6,687
  Vice President, Chief Financial        2002     204,839      --       20,000             6,522
  Officer, Treasurer, and Secretary      2001     213,077      --       25,000             6,112
</Table>

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

(1) Represents premiums paid for life insurance and long-term disability
    policies of which the Company is not the beneficiary, flexible spending
    contributions toward health care costs not covered by Company plans, and the
    ATMI 401(k) Profit Sharing Plan Company match received by the executive in
    that year.

                                        9


OPTION GRANTS

     The following table sets forth certain information with respect to stock
options granted to the Named Executive Officers during the year ended December
31, 2003.

                       OPTION GRANTS IN LAST FISCAL YEAR

<Table>
<Caption>
                                                                                    POTENTIAL REALIZABLE
                                                                                      VALUES AT ASSUMED
                            NUMBER OF      PERCENTAGE                               ANNUAL RATES OF STOCK
                            SECURITIES      OF TOTAL                                 PRICE APPRECIATION
                            UNDERLYING   OPTIONS GRANTED   EXERCISE                  FOR OPTION TERM(2)
                             OPTIONS      TO EMPLOYEES     PRICE PER   EXPIRATION   ---------------------
NAME                        GRANTED(1)   IN FISCAL 2003      SHARE        DATE         5%         10%
- ----                        ----------   ---------------   ---------   ----------   --------   ----------
                                                                             
Eugene G. Banucci.........    60,000          6.59%         $18.52      1/02/13     $698,828   $1,770,967
Douglas A. Neugold........    60,000          6.59%         $18.52      1/02/13     $698,828   $1,770,967
Daniel P. Sharkey.........    30,000          3.29%         $18.52      1/02/13     $349,414   $  885,483
</Table>

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

(1) Options granted vest ratably over five years on each of the first five
    anniversary dates of the grant date.

(2) The potential realizable value is the pre-tax gain that an option holder
    would realize at the time of the option expiration date, if (a) he or she
    exercised all of the options on their expiration date, and (b) the Company's
    stock price grew between the date of grant and the exercise date at the
    annual rate assumed in the column. This pre-tax gain is calculated by
    multiplying the number of options by the difference between the assumed
    stock price on the option expiration date and the option exercise price. The
    hypothetical values reflected in this table represent assumed rates of
    appreciation only; these assumed rates are set by SEC rules and, therefore,
    are not intended to forecast future price appreciation, if any, of the
    Company's common stock. Actual gains, if any, on stock option exercises and
    common stock holdings are dependent on, among other factors, the future
    performance of the common stock and overall stock market conditions. There
    can be no assurance that the amounts reflected in this table will be
    achieved.

OPTION EXERCISES AND VALUES FOR FISCAL 2003

     The following table sets forth information concerning option exercises and
option holdings as of December 31, 2003 with respect to the Named Executive
Officers.

                AGGREGATED OPTION EXERCISES IN LAST FISCAL YEAR
                       AND FISCAL YEAR-END OPTION VALUES

<Table>
<Caption>
                                                      NUMBER OF SECURITIES UNDERLYING          VALUE OF UNEXERCISED
                                                          UNEXERCISED OPTIONS AT              IN-THE-MONEY OPTIONS AT
                           SHARES                            DECEMBER 31, 2003                 DECEMBER 31, 2003(1)
                         ACQUIRED ON      VALUE      ---------------------------------   ---------------------------------
NAME                     EXERCISE(#)   REALIZED($)   EXERCISABLE(#)   UNEXERCISABLE(#)   EXERCISABLE($)   UNEXERCISABLE($)
- ----                     -----------   -----------   --------------   ----------------   --------------   ----------------
                                                                                        
Eugene G. Banucci......     6,000       $ 86,520        230,000           175,000           $773,275          $504,600
Douglas A. Neugold.....        --             --        115,000           153,000           $342,775          $215,100
Daniel P. Sharkey......    22,500       $545,565        116,000            79,000           $154,520          $486,760
</Table>

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

(1) Based on $23.21 per share, the fair market value of the Company's Common
    Stock as of December 31, 2003, minus the exercise price per share of the
    options.

EMPLOYMENT AGREEMENTS

     The Company entered into employment agreements with Eugene G. Banucci and
Daniel P. Sharkey, effective October 10, 1997, and Douglas A. Neugold, effective
April 26, 2000. Pursuant to the agreements, Dr. Banucci will act as Chief
Executive Officer and Chairman of the Board of the Company, Mr. Neugold will act
as President of the Company, and Mr. Sharkey will act as Vice President, Chief
Financial Officer and Treasurer of the Company, for certain annual base
salaries. Salaries are subject to increase from time to time to take into
account appropriate cost of living adjustments and general compensation
increases based on

                                        10


performance, at the discretion of the Board of Directors. Each employee will
also be eligible to receive additional compensation, including awards of
performance bonuses at levels commensurate with other employees of the Company
of equivalent position and grants of employee stock options, in each case in the
discretion of the Compensation Committee of the Board of Directors.

     The employment agreements each were for an initial term of two years, after
which employment continues at will, subject to the terms of the agreements. Each
of the employment agreements expires on the earliest to occur of (i) the death
of the employee, (ii) the termination of the agreement by the Company because of
the incapacity of the employee, (iii) the termination of the agreement by the
Company with or without cause, or (iv) the termination of the agreement by the
employee for just cause. Under the terms of the agreements, if the Company
terminates the employee without cause, or if the employee terminates the
agreement for just cause, the Company will pay the employee his annual base
salary then in effect for a period of 18 months after termination in the case of
Dr. Banucci, for a period of 12 months after termination in the case of Mr.
Neugold, and for a period of nine months after termination in the case of Mr.
Sharkey. The Company will also provide the employee during such period with
medical, dental, life, and disability insurance benefits on the same basis the
Company would have provided the employee the benefits during such period had he
continued to be an employee of the Company.

     The employment agreements also provide that any termination associated with
a change in control of the Company (including resignation by the employee for
just cause, such as a significant decrease in the employee's duties or
authority) would result in the acceleration of vesting of options granted to
them. In addition, the employees would be entitled to any bonuses under any
bonus plans then in effect as if fully earned. Benefits payable under the
agreements upon a change in control may subject the employee to an excise tax as
"excess parachute payments" under Section 280G of the Internal Revenue Code of
1986, as amended. The Company will reimburse the employee for all excise taxes
paid, but the reimbursement will constitute an excess parachute payment and will
be subject to further excise tax. Such further excise tax will trigger further
reimbursement by the Company. The Company will not be allowed to take a
deduction for federal income tax purposes for the excess parachute payments.

COMPENSATION COMMITTEE INTERLOCKS AND INSIDER PARTICIPATION

     No interlocking relationship exists between any of the Company's executive
officers, members of the Company's Board of Directors, or Compensation Committee
and any other company's executive officers, Board of Directors, or compensation
committee.

BOARD COMPENSATION COMMITTEE REPORT ON EXECUTIVE COMPENSATION

     The following report of the Compensation Committee and the performance
graph included elsewhere in this proxy statement do not constitute soliciting
material and should not be deemed filed or incorporated by reference into any
other Company filing under the Securities Act of 1933 or the Securities Exchange
Act of 1934, except to the extent the Company specifically incorporates this
Report or the performance graphs by reference therein.

     The Compensation Committee, which is composed of independent directors of
the Company, is responsible for administering the Company's executive
compensation program and reviewing and making recommendations to the Board of
Directors with respect to the compensation of executive officers and other
senior management, and the Company's overall compensation policy. In connection
with such responsibilities, the Compensation Committee has authority to
administer the Company's 1987, 1995, 1997, 1998, 2000, and 2003 Stock Plans,
including the approving the grant of stock options and other awards thereunder.
All other actions of the Compensation Committee are subject to the approval of
the Board of Directors.

     The Company's executive compensation program is intended to attract and
retain talented executives and senior management by offering competitive
compensation opportunities. Furthermore, the Company's compensation program is
designed to motivate and reward high-performing individuals based on variable
compensation tied to overall corporate, separate business unit, individual
performance, and the creation of stockholder value. The Company's philosophy is
that the combination of performance-based and stock-based
                                        11


compensation serves to maximize annual and long-term results and, ultimately,
stockholder value. The components of the Company's executive compensation
program include base salary, annual cash incentives, long-term stock option and
restricted stock incentives. The Compensation Committee has discretion as to the
composition and components awarded in a particular year to each executive
officer.

Components of Executive Compensation

     Base Salary.  The Compensation Committee annually reviews senior officers'
base salaries. The Compensation Committee evaluates management's recommendations
based on the results achieved by each senior officer relative to the assigned
goals of the recently completed year, as well as competitive salary practices of
other similar companies.

     Annual Incentives.  Annual incentives are designed to provide senior
officers with a potential cash award based on the achievement of annual
financial and operating objectives. These objectives and potential award amounts
are approved by the Compensation Committee and the Board of Directors on an
annual basis in advance, and are based upon operating plans approved by the
Board of Directors. The Compensation Committee approves specific objectives for
each senior officer. In 2003, these objectives included the performance of
business divisions, the achievement of budgeted financial performance, and the
successful completion of certain management objectives and strategic
transactions. Based on the financial performance of the Company, no cash bonuses
were awarded to the senior executive officers for 2003.

     Long-term Incentives.  The Compensation Committee may also recommend to the
Board of Directors the grant to senior officers of stock options and restricted
stock under the Company's 1995, 1997, 1998, 2000, and 2003 Stock Plans. These
options and restricted stock awards, which vest over time, are awarded to senior
officers based on their continued contribution to the Company's achievement of
financial and operating objectives. These awards are designed to align the
interests of the Company's senior officers with the interests of the Company's
stockholders, and to motivate the Company's senior officers to remain focused on
the overall long-term performance of the Company. In 2003, executive officers
received grants of non-qualified stock options. These options were granted at
the fair market value of the Common Stock on the date of grant. The options
become exercisable over a five-year period, and have a ten-year term. In
determining the number of stock options granted to executive officers, the
Compensation Committee took into account position levels, individual
performance, and the number of shares available for issuance under the Company's
1995, 1997, 1998, 2000, and 2003 Stock Plans.

  Chief Executive Officer Compensation

     During 2003, the Company's Chief Executive Officer, Eugene G. Banucci,
participated in the same executive compensation program provided to other
executive officers of the Company as described above. In determining adjustments
to Dr. Banucci's base salary, the Compensation Committee considers market rates
of compensation paid to chief executive officers of comparable companies, the
Company's financial performance, and the fulfillment of a series of objectives
during the previous fiscal year established jointly at the beginning of the year
by the Compensation Committee and the Chief Executive Officer. The objectives
used to determine base salary for fiscal 2003 consisted of organization and
strategy development, merger and acquisition activity, and certain financial
objectives. Over the course of 2003, Dr. Banucci received a base salary of
$410,558, which represented a 9.9% increase over his base salary in 2002. In
December 2003, based upon the Company's financial performance during 2003, the
Compensation Committee authorized no cash bonus for 2003. On January 1, 2003,
Dr. Banucci was granted non-qualified stock options to purchase 60,000 shares of
Common Stock at an exercise price of $18.52 per share, which was the fair market
value of the Common Stock on the date of the grant. The options become
exercisable over a five-year period and have a ten-year term.

                                          Stephen H. Mahle, Chairman
                                          Mark A. Adley
                                          C. Douglas Marsh

                                        12


Stock Performance Graph

     The following graph compares the cumulative total stockholder return on the
Company's Common Stock with the return on the Total Return Index for the Nasdaq
Stock Market (U.S.) and the Nasdaq Electronic Components Stock Index. The
measurement assumes a $100 investment as of December 31, 1998 with all dividends
reinvested. The data presented are on an annual basis for the five years ended
on December 31, 2003.

                            STOCK PERFORMANCE GRAPH

                           [STOCK PERFORMANCE GRAPH]

<Table>
<Caption>
                                         NASDAQ
                             NASDAQ    ELECTRONIC
                              STOCK    COMPONENTS
           DATE     ATMI     MARKET      INDEX
         --------  -------   -------   ----------
                              
         12/31/98  $100.00   $100.00    $100.00
         12/31/99  $130.93   $185.43    $185.99
         12/31/00  $ 77.23   $111.83    $152.85
         12/31/01  $ 94.46   $ 88.76    $104.43
         12/31/02  $ 73.35   $ 61.37    $ 55.92
         12/31/03  $ 91.92   $ 91.75    $107.60
</Table>

                                        13


                 CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS

     There were no relationships or related transactions in fiscal year 2003
which are required by law to be disclosed in this proxy statement.

                         REPORT OF THE AUDIT COMMITTEE

     The following report of the Audit Committee does not constitute soliciting
material and should not be deemed filed or incorporated by reference into any
other Company filing under the Securities Act of 1933 or the Securities Exchange
Act of 1934, except to the extent the Company specifically incorporates this
report by reference therein.

     Pursuant to the charter of the Audit Committee, a copy of which is attached
as Appendix A, the responsibilities of the Audit Committee fall within four
broad categories:

     -  Audit Process.  The Audit Committee reviews the coordination of the
        audit effort between management and the independent auditors, and the
        audit scope and plan of the independent auditors. It appoints the
        independent auditors, reviews their fees, and approves any change in
        independent auditors. It obtains from the independent auditors a written
        statement concerning their independence and reviews their independence.

     -  Financial Reporting.  The Audit Committee reviews the independent
        auditors' report and opinion on the Company's financial statements, and
        reviews with management the financial statements; it recommends approval
        of the financial statements to the Board of Directors. It reviews the
        independent auditors' summary of significant accounting, auditing, and
        internal control issues and other matters related to the conduct of the
        audit. It reviews with management and the independent auditors
        accounting policy changes and new accounting or reporting standards. It
        reviews with the independent auditors their judgments about the quality
        and appropriateness, not just the acceptability, of the Company's
        accounting principles and estimates. The Audit Committee, or the
        Chairman of the Committee, reviews with management and the independent
        auditors the Company's interim financial statements.

     -  Internal Controls.  The Audit Committee reviews with management and the
        independent auditors the adequacy of the Company's system of internal
        accounting controls, and reviews reports from management summarizing
        findings and the resolution of previously reported control issues. It
        also reviews any significant risks or exposures.

     -  Complaints and Procedures.  The Audit Committee establishes procedures
        for the receipt, retention and treatment of complaints received by the
        Company regarding accounting, internal accounting controls, or auditing
        matters, and the confidential, anonymous submission by employees of
        concerns regarding questionable accounting or auditing matters, and
        reviews calls to an ethics hotline to address any such complaints or
        concerns.

     -  Other Matters.  The Audit Committee monitors compliance with regulatory
        matters. It reviews the Audit Committee charter on an annual basis. It
        meets with management and the independent auditors at least annually in
        separate executive sessions.

     Please refer to the Audit Committee charter in Appendix A for a more
comprehensive description of the role and responsibilities of the Audit
Committee.

     In connection with its duties, the Audit Committee has taken the following
actions:

     -  It has reviewed and discussed the consolidated audited financial
        statements with management, which has responsibility for the preparation
        of the financial statements in accordance with generally accepted
        accounting principles.

     -  It has discussed with the independent auditors, which are responsible
        for expressing an opinion on the financial statements in accordance with
        generally accepted auditing standards, the matters required to

                                        14


        be discussed by Statement on Auditing Standards No. 61, "Communication
        with Audit Committees," as amended.

     -  It has received from the independent auditors the written disclosures
        describing any relationships between the independent auditors and the
        Company, and the letter confirming their independence required by
        Independence Standards, Board Standard No. 1, "Independence Discussions
        with Audit Committees," and has discussed with the independent auditors
        matters relating to their independence.

     -  It has reviewed fees paid to the independent auditors for audit
        services, as compared with fees paid for other services rendered by the
        independent auditors, discussed the effect of fees paid for other
        services on the auditors' independence, and determined that such fees do
        not impair the independence of the auditors.

     Based on its review and discussions described above, the Audit Committee
recommended to the Board of Directors that the audited financial statements of
the Company for the year ended December 31, 2003 be included in the Company's
Annual Report on Form 10-K for filing with the SEC.

                                          Robert S. Hillas, Chairman
                                          Mark A. Adley
                                          Michael J. Yomazzo

                                 PROPOSAL NO. 2

              RATIFICATION OF APPOINTMENT OF INDEPENDENT AUDITORS

     The Audit Committee of the Board of Directors has appointed Ernst & Young
LLP as independent auditors to audit the consolidated financial statements of
the Company for the fiscal year ending December 31, 2004, and has determined
that it would be desirable to request that the stockholders ratify such
appointment. Ernst & Young LLP served as the Company's independent auditors for
the fiscal year ended December 31, 2003, and has reported on the Company's
consolidated financial statements for such year. Representatives of Ernst &
Young LLP are expected to be present at the Annual Meeting, will have the
opportunity to make a statement if they desire to do so, and will be available
to respond to appropriate questions from stockholders.

     While stockholder ratification is not required for the appointment of Ernst
& Young LLP, since the Audit Committee of the Board of Directors has the
responsibility for appointing the Company's independent auditors, the
appointment is being submitted for ratification at the Annual Meeting with a
view toward soliciting the stockholders' opinions, which the Audit Committee of
the Board of Directors will take into consideration in future deliberations.

                                        15


FEES TO INDEPENDENT AUDITORS FOR FISCAL 2003 AND 2002

     The following table presents fees for professional services rendered by
Ernst & Young LLP for the audit of the Company's consolidated financial
statements for fiscal 2003 and fiscal 2002 and fees billed for audit-related
services, tax services and all other services rendered by Ernst & Young LLP for
fiscal 2003 and fiscal 2002. In accordance with applicable policy, all services
rendered by Ernst & Young LLP, after the provision of services under the "de
minimis" safe harbor, were approved by the Audit Committee.

<Table>
<Caption>
                                                             FISCAL 2003   FISCAL 2002
                                                             -----------   -----------
                                                                  (IN THOUSANDS)
                                                                     
(1) Audit fees(a)..........................................     $508          $508
(2) Audit-related fees(b)..................................     $ 18          $ 38
(3) Tax fees(c)............................................     $291          $405
(4) All other fees(d)......................................     $ 12          $  0
                                                                ----          ----
     Total.................................................     $829          $951
                                                                ====          ====
</Table>

- ---------------
(a)  For the audit of ATMI's annual financial statements, the reviews of the
     financial statements included in ATMI's reports on Form 10-Q, and for
     services that are normally provided by the independent auditors in
     connection with statutory and regulatory filings or engagements for those
     fiscal years.

(b)  For assurance and related services that are related to the performance of
     the audit or review of the Company's financial statements, other than the
     fees disclosed in the foregoing paragraph. These fees related primarily to
     audits of the 401(k) plan and internal controls reviews.

(c)  For tax compliance, tax advice, and tax planning.

(d)  For assistance with planning associated with Section 404 of the
     Sarbanes-Oxley Act.

PRE-APPROVAL OF AUDIT AND NON-AUDIT SERVICES

     Under the Audit and Non-Audit Services Pre-Approval Policy, as adopted by
the Audit Committee in 2003, the Audit Committee must pre-approve all audit and
non-audit services provided by the independent auditors. The policy, as
described below, sets forth the procedures and conditions for such pre-approval
of services to be performed by the independent auditors. The policy utilizes
both a framework of general pre-approval for certain specified services and
specific pre-approval for all other services.

     In the fourth quarter of each year, the Audit Committee is asked to
pre-approve the engagement of the independent auditors, and the projected fees,
for audit services, audit-related services (assurance and related services that
are reasonably related to the performance of the auditor's review of the
financial statements or that are traditionally performed by the independent
auditors) and tax services (such as tax compliance, tax planning and tax advice)
for the following year.

     The fee amounts approved at such fourth quarter meeting are updated to the
extent necessary at the regularly scheduled meetings of the Audit Committee in
the following year. Additional pre-approval is required before actual fees for
any service can exceed the originally pre-approved amount.

     If the Company wants to engage the independent auditors for other services,
that are not considered subject to general pre-approval as described above, then
the Audit Committee must approve such specific engagement as well as the
projected fees. Additional pre-approval is required before any fees can exceed
those fees approved for any such specifically-approved services.

     If the Company wishes to engage the independent auditors for additional
services that have not been generally pre-approved as described above, then such
engagement will be presented to the Audit Committee for pre-approval at its next
regularly scheduled meeting. If the timing of the project requires an expedited
decision, then the Company may ask the Chairman of the Audit Committee to
pre-approve such engagement. Any such pre-approval by the Chairman is then
reported to the other Committee members at the next

                                        16


Committee meeting. In any event, pre-approval of any engagement by the Audit
Committee or the Chairman of the Audit Committee is required before the
independent auditors may commence any engagement.

     In 2003, there were no fees paid to Ernst & Young LLP under a "de minimis"
exception to the rules that waives pre-approval for certain non-audit services.

     The Audit Committee monitored the activities and performance of Ernst &
Young LLP, including the audit scope, audit fees, auditor independence matters,
and the extent to which Ernst & Young LLP was retained to perform non-audit
services. In its review of non-audit service fees, the Audit Committee
considered, among other things, the possible effect of the performance of such
services on Ernst & Young LLP's independence. After discussion with management
and Ernst & Young LLP, the Audit Committee considers the audit and non-audit
services provided by Ernst & Young LLP to have been consistent with maintaining
its independence.

WILL REPRESENTATIVES OF ERNST & YOUNG LLP BE PRESENT AT THE ANNUAL MEETING?

     Representatives of Ernst & Young LLP are expected to be present at the
Annual Meeting and will be allowed to make a statement if they wish.
Additionally, they will be available to respond to appropriate questions from
stockholders during the Annual Meeting.

                                 OTHER MATTERS

     The Board of Directors knows of no other matters to be brought before the
Annual Meeting. If, however, any other business should properly come before the
Annual Meeting, the persons named in the accompanying proxy will vote the proxy
as in their discretion they may deem appropriate, unless they are directed by
the proxy to do otherwise.

                             ADDITIONAL INFORMATION

     "Householding" of Proxy Materials.  The SEC has adopted rules that permit
companies and intermediaries such as brokers to satisfy delivery requirements
for proxy statements with respect to two or more stockholders sharing the same
address by delivering a single proxy statement addressed to those stockholders.
This process, which is commonly referred to as "householding," potentially
provides extra convenience for stockholders and cost savings for companies. The
Company and some brokers household proxy materials, delivering a single proxy
statement to multiple stockholders sharing an address unless contrary
instructions have been received from the affected stockholders. Once you have
received notice from your broker or us that they or we will be householding
materials to your address, householding will continue until you are notified
otherwise or until you revoke your consent. If, at any time, you no longer wish
to participate in householding and would prefer to receive a separate proxy
statement, or if you are receiving multiple copies of the proxy statement and
wish to receive only one, please notify your broker if your shares are held in a
brokerage account or us if you hold registered shares. You may notify us by
sending a written request to Investor Relations, ATMI, Inc., 7 Commerce Drive,
Danbury, Connecticut 06810.

     Advance Notice Procedures.  Under our bylaws, any stockholder of record of
the Company may nominate candidates for election to the Board of Directors, or
present other business at an annual meeting, if a written notice is delivered to
the Secretary of the Company at the Company's principal executive offices not
less than 60 days, nor more than 90 days, prior to the first anniversary of the
preceding year's annual meeting -- that is, with respect to the annual meeting
of stockholders in 2005, between February 24 and March 26, 2005. Such written
notice must set forth:

     -  As to each person whom the stockholder proposes to nominate for election
        as a director, all information relating to such person that is required
        to be disclosed in solicitations of proxies for election of directors
        pursuant to the Exchange Act (including such person's written consent to
        being named in the proxy statement as a nominee and to serving as a
        director if elected); and

                                        17


     -  As to any other business to be brought before the meeting: (a) a brief
        description of the business; (b) the reasons for conducting such
        business; and (c) any material interest in such business of such
        stockholder and the beneficial owner, if any, on whose behalf the
        proposal is made; and as to the stockholder and the beneficial owner, if
        any, on whose behalf the nomination or proposal is made: (i) the name
        and address of such stockholder and such beneficial owner; and (ii) the
        number of shares of Common Stock that are held of record by such
        stockholder, and owned beneficially by such beneficial owner.

     These requirements are separate from and in addition to the SEC's
requirements that a stockholder must meet in order to have a stockholder
proposal included in the Company's proxy statement.

     Stockholder Proposals for the 2005 Annual Meeting.  Stockholders interested
in submitting a proposal for inclusion in the proxy materials for the annual
meeting of stockholders in 2005 may do so by following the procedures prescribed
in SEC Rule 14a-8 under the Exchange Act. To be eligible for inclusion,
stockholder proposals must be received by the Company no later than December 23,
2004. Proposals should be sent to Investor Relations, ATMI, Inc., 7 Commerce
Drive, Danbury, Connecticut 06810.

     Proxy Solicitation Costs.  The proxies being solicited hereby are being
solicited by the Board of Directors of the Company. The cost of soliciting
proxies in the enclosed form will be borne by the Company. Officers and regular
employees of the Company may, but without compensation other than their regular
compensation, solicit proxies by further mailing or personal conversations, or
by telephone, telex, facsimile or electronic means. We will, upon request,
reimburse brokerage firms and others for their reasonable expenses in forwarding
solicitation material to the beneficial owners of stock.

                                        18


                                                                      APPENDIX A

                                   ATMI, INC.

                            AUDIT COMMITTEE CHARTER

THE PURPOSE OF THE AUDIT COMMITTEE

     The purpose of the Audit Committee is to represent and assist the Board of
Directors in its general oversight of the Company's accounting and financial
reporting processes, audits of the financial statements, and internal control
and audit functions. Management is responsible for (a) the preparation,
presentation and integrity of the company's financial statements; (b) accounting
and financial reporting principles; and (c) the company's internal controls and
procedures designed to promote compliance with accounting standards and
applicable laws and regulations. The company's independent auditing firm is
responsible for performing an independent audit of the consolidated financial
statements in accordance with generally accepted auditing standards.

     The Audit Committee members are not professional accountants or auditors
and their functions are not intended to duplicate or to certify the activities
of management and the independent auditors, nor can the Committee certify that
the independent auditors are "independent" under applicable rules. The Audit
Committee serves a board level oversight role where it oversees the relationship
with the independent auditors, as set forth in this charter, and provides
advice, counsel and general direction, as it deems appropriate, to management
and the independent auditors on the basis of the information it receives,
discussions with the independent auditors, and the experience of the Committee's
members in business, financial and accounting matters.

MEMBERSHIP

     The Audit Committee is comprised of at least three directors determined by
the Board of Directors to meet the independence and financial literacy
requirements of The NASDAQ Stock Market, Inc. ("NASDAQ") and applicable federal
law. Appointment to the Committee, including the designation of the Chair of the
Committee and the designation of any Committee members as "audit committee
financial experts", shall be made on an annual basis by the full Board upon
recommendation of the Nominating and Corporate Governance Committee.

RESPONSIBILITIES

     The Audit Committee:

     - Is directly responsible for the appointment, replacement, compensation,
       and oversight of the work of the independent auditors. The independent
       auditors shall report directly to the Audit Committee.

     - Shall obtain and review, at least annually, a report by the independent
       auditors describing: (a) the firm's internal quality-control procedure;
       (b) any material issues raised by the most recent internal
       quality-control review, or peer review, of the firm, or by any inquiry or
       investigation by governmental or professional authorities, within the
       preceding five years, respecting one or more independent audits carried
       out by the firm, and any steps taken to deal with any such issues.

     - Reviews and discusses the written statement from the independent auditor
       concerning any relationship between the independent auditors and the
       company or any other relationships that may adversely affect the
       independence of the independent auditors, and, based on such review,
       assesses the independence of the auditor.

     - Establishes policies and procedures for the review and pre-approval by
       the Committee of all auditing services and permissible non-audit services
       (including the fees and terms thereof) to be performed by the independent
       auditors, with exceptions provided for de minimis amounts under certain
       circumstances as described by law. The Committee may delegate
       pre-approval authority to a member of the

                                       A-1


       Audit Committee. The decisions of any Committee member to whom
       pre-approval authority is delegated must be presented to the full
       Committee at its next scheduled meeting.

     - Reviews and discusses with the independent auditors: (a) its audit plans,
       and audit procedures, including the scope, fees and timing of the audit;
       (b) the results of the annual audit examination and accompanying
       management letters; and (c) the results of the independent auditors'
       procedures with respect to interim periods.

     - Reviews and discusses reports from the independent auditors on (a) all
       critical accounting policies and practices used by the Company, (b)
       alternative accounting treatments within GAAP related to material items
       that have been discussed with management, including the ramifications of
       the use of the alternative treatments and the treatment preferred by the
       independent auditor, and (c) other material written communications
       between the independent auditors and management.

     - Reviews with the independent auditors its judgments as to the quality,
       not just the acceptability, of the company's accounting principles and
       such matters as are required to be discussed with the Committee under
       generally accepted auditing standards.

     - Discusses with management and the independent auditors quarterly earnings
       press releases, including the interim financial information, reviews the
       quarterly financial statements, including "Management's Discussion and
       Analysis of Financial Condition and Results of Operations" prior to the
       filing of the Company's Quarterly Report on Form 10-Q, reviews the
       year-end audited financial statements and "Management's Discussion and
       Analysis of Financial Condition and Results of Operations" and, if deemed
       appropriate, recommends to the Board of Directors that the audited
       financial statements be included in the Annual Report on Form 10-K for
       the year.

     - Reviews and discusses with management and the independent auditors
       various topics and events that may have significant financial impact on
       the company or that are the subject of discussions between management and
       the independent auditors.

     - Reviews and discusses with management the company's major financial risk
       exposures and the steps management has taken to monitor and control such
       exposures.

     - Reviews and has prior-approval authority for related-party transactions
       (as defined in the relevant NASDAQ requirements).

     - Reviews and discusses with management and the independent auditors: (a)
       the adequacy and effectiveness of the company's internal controls
       (including any significant deficiencies and significant changes in
       internal controls reported to the Committee by the independent auditors
       or management); (b) the company's internal audit procedures; and (c) the
       adequacy and effectiveness of the company's disclosure controls and
       procedures, and management reports thereon.

     - Reviews the use of auditors other than the independent auditors in cases
       such as management's request for second opinions.

     - Establishes procedures for the receipts, retention and treatment of
       complaints received by the Company regarding accounting, internal
       accounting controls, or auditing matters, and the confidential, anonymous
       submission by employees of concerns regarding questionable accounting or
       auditing matters.

     - Establishes policies for the hiring of employees and former employees of
       the independent auditors.

     - Publishes the report of the Committee required by the rules of the
       Securities and Exchange Commission to be included in the company's annual
       proxy statement.

     - Reviews periodically the Company's program for monitoring compliance with
       the ATMI Securities Trading Policy and the Foreign Corrupt Practices Act
       of 1977, including violations of such policy or act.

                                       A-2


     - Reviews the Company's legal compliance programs and policies, and reports
       received from regulatory agencies; monitoring the results of compliance
       efforts.

     - Reviews periodically the Company's code of ethics that applies to all of
       its directors, officers and employees, monitoring the results of
       compliance efforts.

     - When appropriate, designates one or more of its members to perform
       certain of its duties on its behalf, subject to such reporting to or
       ratification by the Committee as the Committee shall direct.

     The Audit Committee will engage in an annual self-assessment with the goal
of continuing improvement, and will annually review and reassess the adequacy of
its charter, and recommends any changes to the full Board.

     The Audit Committee shall have the authority to engage independent legal,
accounting and other advisers, as it determines necessary to carry out its
duties. The Audit Committee shall have sole authority to approve related fees
and retention terms.

     The Audit Committee shall meet at such times and places as the Audit
Committee shall determine. The Audit Committee shall meet in executive session
with the independent auditors and management periodically. The Chairman of the
Audit Committee shall report on Audit Committee activities to the full Board.

     The Chairman of the Audit Committee is to be contacted directly by
independent auditors (1) to review items of a sensitive nature that can impact
the accuracy of financial reporting or (2) to discuss significant issues
relative to the overall Board responsibility that have been communicated to
management but, in their judgment, may warrant follow-up by the Audit Committee.

                                       A-3


              PROXY SOLICITED ON BEHALF OF THE BOARD OF DIRECTORS
                     FOR THE ANNUAL MEETING OF STOCKHOLDERS
                                ON MAY 25, 2004

     The undersigned stockholder of ATMI, Inc. (the "Company") hereby revokes
all prior proxies and hereby appoints each of Eugene G. Banucci and Daniel P.
Sharkey as a proxy for the undersigned, each with full power of substitution, to
vote all shares of Common Stock of the Company which the undersigned is entitled
to vote at the Company's annual meeting of stockholders for the fiscal year
ended December 31, 2003, to be held at ATMI, Inc., 6 Commerce Drive, Danbury,
Connecticut 06810, on May 25, 2004, at 10:00 a.m., local time, and at any
adjournment thereof, and the undersigned authorizes and instructs such proxies
or their substitutes to vote as follows:

<Table>
                                                  
1.   ELECTION OF DIRECTORS: To elect the nominees listed below to the board of directors for the term
     set forth in Proposal No. 1 of the Proxy Statement:

     FOR THE NOMINEES LISTED BELOW                      WITHHOLD AUTHORITY
     (except as marked to the contrary below)  [ ]      to vote for the nominee listed below  [ ]

     (INSTRUCTION: TO WITHHOLD AUTHORITY TO VOTE FOR THE NOMINEE, STRIKE A LINE THROUGH THE NOMINEE'S
     NAME IN THE LIST BELOW.)

     Robert S. Hillas                                [ ]
     Michael J. Yomazzo                              [ ]

2.   RATIFICATION OF APPOINTMENT OF INDEPENDENT AUDITORS: To ratify the appointment of Ernst & Young
     LLP as the Company's independent auditors for the fiscal year ending December 31, 2004:

                    FOR  [ ]        AGAINST  [ ]        ABSTAIN  [ ]
</Table>

and in their discretion upon any other matter that may properly come before the
meeting or any postponement or adjournment thereof.

                                SEE REVERSE SIDE
          CONTINUED AND TO BE SIGNED ON REVERSE SIDE SEE REVERSE SIDE


     THIS PROXY WHEN PROPERLY EXECUTED WILL BE VOTED IN THE MANNER DIRECTED
HEREIN BY THE UNDERSIGNED STOCKHOLDER. IN THE ABSENCE OF DIRECTION, THIS PROXY
WILL BE VOTED FOR PROPOSALS 1 AND 2 AND, IN ACCORDANCE WITH THE JUDGMENT OF THE
PROXIES, FOR OR AGAINST ANY OTHER MATTER THAT MAY PROPERLY COME BEFORE THE
ANNUAL MEETING AND ANY POSTPONEMENT OR ADJOURNMENT THEREOF.

     Receipt of the Notice of Annual Meeting and of the Proxy Statement and the
Company's 2003 Annual Report to stockholders accompanying the same is hereby
acknowledged.

     PLEASE DATE, SIGN AND RETURN THIS PROXY PROMPTLY USING THE ENCLOSED
ENVELOPE.

           [ ]  MARK HERE FOR ADDRESS CHANGE, AND NOTE AT LEFT BELOW.

                                          Dated: ------------------------- ,
                                          2004

                                          --------------------------------------
                                          (Signature of Stockholder)

                                          --------------------------------------
                                          (Signature of Stockholder)

                                          Please sign exactly as your name(s)
                                          appears on your stock certificate. If
                                          signing as attorney, executor,
                                          administrator, trustee or guardian,
                                          please indicate the capacity in which
                                          signing. When signing as joint
                                          tenants, all parties to the joint
                                          tenancy must sign. When the proxy is
                                          given by a corporation, it should be
                                          signed by an authorized officer.