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                                                                   EXHIBIT 10.16

                           FOURTH AMENDMENT OF LEASE

THIS FOURTH AMENDMENT OF LEASE made as of December 4, 2000 by and between 50
INWOOD ROAD LIMITED PARTNERSHIP, a limited partnership, organized and existing
under the laws of the State of Connecticut having its place of business at c/o
DaQui Belding Partners Inc. at 175 capital Boulevard, Corporate Ridge, Rocky
Hill, Connecticut 06067 (hereinafter referred to as the "Landlord"), and PROTON
ENERGY SYSTEMS, INC. with its principal place of business at 50 Inwood Road,
Inwood Business Park, Rocky Hill, Connecticut 06067 (hereinafter referred to as
the "Tenant")

                                  WITNESSETH:

     WHEREAS, Landlord is the owner of certain real estate known as 50 Inwood
Road, Inwood Business Park, located in Rocky Hill, Connecticut (hereinafter
called the "Premises");

     WHEREAS, Landlord and Tenant have entered into a lease of a portion of the
Premises by Lease dated May 27, 1997 (the "Lease") amended on January 29, 1998
(The "First Amendment of lease"); amended on March 1, 1999 (the "Second
Amendment of Lease"); and amended as of April 9, 1999 (the "Third Amendment of
Lease")

     WHEREAS, Landlord and Tenant have agreed to modify the terms of the Lease
effective July 1, 2001.

     NOW, THEREFORE, in consideration of the mutual promises contained herein
and for other valuable consideration, the receipt of which is hereby
acknowledged, and subject to the approval of the Fourth Amendment of lease by
Landlord's lenders, Landlord and Tenant hereby covenant and agree as follows:

     1.   Definition of Terms.  All defined, capitalized terms used herein shall
          -------------------
          have the same meanings as are ascribed to them in the Lease.

     2.   Term of the Extension Term.  The Extension Term shall commence on July
          --------------------------
          1, 2001 (the "Extension Term Commencement Date") and shall terminate
          on June 30, 2004.  All references in the lease to the "Expiration
          Date" shall hereafter refer to June 30, 2004.  Tenant hereby
          acknowledges that, upon the execution of this Amendment by both
          parties hereto, the option to extend the Term of the Lease, as
          provided in Section 38 of the lease, shall have been exercised.
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3.        Section 1.5 is modified by adding the following:

                                                             Annual Rate
                                                             Per Rentable
          Lease Year   Annual Base Rent   Monthly Payments   Square Foot
          ----------   ----------------   ----------------   -----------

          7/01 - 6/04    $179,089.50         $14,924.12         $8.95

4.        Every provision, covenant, condition, obligation, right and power
          contained in and under the lease shall continue in full force and
          effect, affected by this Fourth Amendment only to the extent of the
          amendments set forth above.

5.        All provisions of this Fourth Amendment shall be binding upon and
          inure to the benefit of the parties hereto and their respective
          successors and assigns.

IN WITNESS WHEREOF, Landlord and Tenant have caused this Fourth Amendment to be
duly executed as the day and year first written above.

WITNESSED BY:                   LANDLORD:

                                50 INWOOD ROAD
________________                LIMITED PARTNERSHIP

                                BY: INWOOD PROPERTIES, LLC
________________                Its General Partner

                                BY: Cromwell Avenue Management Co.
                                Its Managing Member

                                BY:____________________
                                    Paul J. Daqui
                                    President

________________                TENANT:

                                PROTON ENERGY SYSTEMS, INC.

________________                BY:_________________
                                    Robert Friedland
                                    Vice President Operations