EXHIBIT 4 (bb)


                                                                             216
<PAGE>

                                   ----------

                             THE TIREX CORPORATION

                                   ----------

                                Amendment to 10%
                       Convertible Subordinated Debenture,
                       Securities Purchase Agreement, and
                          Registration Rights Agreement

      This Amendment is made as of September 8, 1998 between ___________________
(the "Debentureholders") and The Tirex Corporation ("Tirex")

      Whereas,  on April 9, 1998,  the  Debentureholders  purchased from Tirex a
unit (the  "Unit")  of Tirex's  securities,  consisting  of one 10%  Convertible
Subordinated Debenture (the "Debenture") in the principal amount of US $250,000,
redeemable  at 125% of face  value  plus  interest  one  year  from  the date of
issuance  (the  "Maturity  Date"),  and warrants to purchase an aggregate of one
million  (1,000,000) shares of the common stock of Tirex at a per share price of
$.001 (the "Warrants");

      Whereas,  Tirex was, at the time of the Debentureholder's  purchase of the
Unit, and currently remains, a thinly capitalized development stage Company;

      Whereas, the purchase and sale of the securities  comprising the Unit were
effected  pursuant  to  the  terms  of  a  securities  purchase  agreement  (the
"Securities  Purchase  Agreement")  and a  registration  rights  agreement  (the
"Registration Rights Agreement") between Tirex and the  Debentureholders,  dated
April 9, 1998;

      Whereas,  Paragraph (b) of Article 2 of the Registration  Rights Agreement
provides for liquidated  damages (the "Liquidated  Damages") to be paid by Tirex
for each  thirty-day  period  subsequent  to  September 8, 1998 during which the
registration  statement  filed  by  Tirex  on May 21,  1998  (the  "Registration
Statement"), registering the common stock underlying the Debenture and Warrants,
has not been declared effective,  unless the delay in achieving effectiveness is
caused by the Securities and Exchange Commission;

      Whereas,  because of unforeseen  circumstances,  and  notwithstanding  the
continuing  best efforts of Tirex to have the  Registration  Statement  declared
effective as expeditiously as possible,  as of the date hereof, the Registration
Statement  has not yet been  declared  effective  and the Company is not able to
state at this  time,  when  the said  Registration  Statement  will be  declared
effective;


                                                                             217
<PAGE>

      Whereas, the  Debentureholders  believe that it would not be in their best
interests,  as investors in Tirex, to require Tirex to: (i) carry the Debentures
on its financial statements as a short-term liability due and payable within the
current fiscal year, thereby diminishing Tirex's ability to obtain bank or other
short-term  debt  financing;  or (ii) pay the Liquidated  Damages,  because such
obligation would constitute a significant  financial burden which, at this stage
of the  development of Tirex's  business,  could cause  substantial and possibly
irreparable  adverse  financial  consequences  which could seriously  delay, and
possibly destroy,  Tirex's ability to commence its TCS-1 Plant manufacturing and
TCS-1 Plant Operating businesses, on commercial bases;

      Whereas,  subsequent to April 9, 1998,  the parties  amended the Debenture
and the Securities Purchase Agreement to provide that the Debenture would not be
convertible  during the period  commencing  on the date  preceding the filing by
Tirex of the  Registration  Statement and terminating on the earlier of: (i) 120
days from the  filing of the  Registration  Statement  with the  Securities  and
Exchange  Commission;  or  (ii)  the  effective  date of the  said  Registration
Statement;

      Whereas, the parties believe it is in their mutual best interests:  (i) to
extend the Maturity Date of the Debenture to December 31, 1999; (ii) to have the
Debenture   (including  all  interest   accrued  thereon  through  the  date  of
conversion)  be  immediately  convertible;  and (iii) in  consideration  for the
accommodations hereby made by the  Debentureholders,  pending the effective date
of the Registration  Statement, to reduce the conversion ratio of the Debenture,
on a monthly  basis,  at a rate of one percent  (1%) of the average  closing bid
price of Tirex's  common  stock for each month  between  September  11, 1998 and
December 11, 1998, and one and one-half  percent (1.5%) per month for each month
between December 12, 1998 and April 11, 1999;

      Whereas,  Tirex has also agreed,  upon  effectiveness  of the Registration
Statement,  to provide  the  Debentureholders  and/or  their  broker,  with such
quantity of  prospectuses  as are  reasonably  necessary to effect the resale of
shares of Tirex common stock underlying the Debentures and Warrants.

      Now,  therefore,  in  consideration  of the  premises  and  of the  mutual
promises hereinafter set forth, the parties agree as follows:

A.  AMENDMENT OF DEBENTURE

1. Maturity Date. The Maturity Date of the Debenture is hereby amended such that
all unpaid  principal and all accrued and unpaid  interest  thereon shall be due
and payable on December 31, 1999.


                                                                             218
<PAGE>

2. Amendment of Article 1. Conversion, Redemption and Registration

      Paragraphs  (a),  (b) and (g) of  Article 1 of the  Debenture  are  hereby
amended so as to read as follows:

      1. CONVERSION, REDEMPTION AND REGISTRATION.

      (a) This  Debenture  is  convertible  in whole or in part at any time into
that number of shares of the Company's  common stock,  par value $.001 per share
("Common Stock") as is obtained by dividing the then unpaid principal face value
of the  Debenture  by an amount equal to a percentage  of  seventy-five  percent
(75%) (the "Conversion Ratio") of the closing bid price of the Common Stock on a
securities exchange or on the National  Association of Securities Dealers,  Inc.
("NASD") Over-the-Counter Electronic Bulletin Board System (the "Market Price"),
on the  trading  date  immediately  preceding  the date  upon  which  notice  of
conversion  is properly  provided to the Company by the holder (the  "Conversion
Date"), subject to reduction as set forth below.

      Until such time as the Registration  Statement on Form SB-2  (Registration
No.  333-53255),  registering  the shares  issuable upon the  conversion of this
Debenture  (the  "Registration  Statement"),  has been declared  effective,  the
Conversion Ratio shall be reduced,  on a monthly basis, at a rate of one percent
(1%) of the Market Price on the date  preceding the  Conversion  Date,  for each
month between September 11, 1998 and December 10, 1998, and at a rate of one and
one-half  percent (1.5%) per month for each month between  December 11, 1998 and
July 10, 1999, as follows:

                     CONVERSION DATE:                      CONVERSION RATIO
                     ----------------                      ----------------

            April 9, 1998 - September 10, 1998                   75%

            September 11, 1998 - October 10, 1998                74%

            October 11, 1998 - November 10, 1998                 73%

            November 11, 1998 - December 10, 1998                72%

            December 11, 1998 - January 10, 1999                 70.5%

            January 11, 1999 - February 10, 1999                 69%

            February 11, 1999 - March  10, 1999                  67.5%

            March 11, 1999 - April 10, 1999                      66%

            April 11, 1999 - May 10, 1999                        64.5%


                                                                             219
<PAGE>

            May 11, 1999 - June 10, 1999                         63%

            June 11, 1999 - July 10, 1999                        61.5%

      If the Common Stock is not listed on any securities exchange at the time a
notice of conversion is issued,  the conversion  price shall be such price as is
determined as the fair and reasonable  price a third party not  affiliated  with
the  Company  would  pay for the  Common  Stock as  determined  by the  Board of
Directors.  At the election of the Payee, all accrued but unpaid interest hereon
may also be converted into Common Stock in the manner  prescribed  herein.  Such
shares of Common Stock are referred to herein as the  "Conversion  Shares." This
Debenture may be partially converted and in case of such partial conversion, the
Company,  upon  surrender  hereof,  will  deliver to the Holder a new  Debenture
representing the principal face value which has not been converted.

      (b) This Debenture is convertible into shares of Common Stock at any time.
The Holder hereof shall have no conversion  rights following  payment in full of
the  principal  and  interest  owed by the  Company  to the Holder  hereof.  The
conversion rights represented by this Debenture may be exercised, in whole or in
part, by the Holder at any time, and from time to time, by delivery of: (i) this
Debenture for cancellation; and (ii) written notice to the Company of the amount
of the Debenture to be converted.  Such delivery  shall be made at the principal
executive  office  of the  Company  (or at such  other  office  or agency of the
Company as it may designate), and may be made by fax with telephone confirmation
of receipt of such fax by the Company's corporate counsel.  Any delivery made by
fax herewith  shall be followed by delivery of the originals of such  documents,
by mail. This Debenture  shall be deemed to have been converted,  in whole or in
part to the extent specified,  immediately prior to the close of business on the
date on which delivery by fax (with telephone  confirmation of receipt) shall be
made. Upon receipt of the foregoing, the Company shall immediately authorize its
transfer agent,  Continental  Stock Transfer & Trust Company,  Inc. to (i) issue
the  Conversion  Shares;  and (ii)  deliver the  certificate  representing  such
Conversion Shares to the Holder as the Holder shall direct.

      (g) The  Company  shall use its best  efforts  to cause  the  Registration
Statement to be declared effective by the SEC as soon as practicable and to keep
such  registration  statement  effective  at all times  until the  delivery of a
prospectus is no longer  required in connection  with the sale of the Conversion
Shares.  Moreover,  if at any time the Registration  Statement is not effective,
the Company intends to file an unrelated  registration  statement,  other than a
registration  statement on Form S-8, under the  Securities  Act, then, not later
than  twenty  days  prior to the  intended  filing  date  for such  registration
statement,  the Company shall give written notice to the Holder of its intention
to file a  registration  statement  and the Holder  shall  have the right,  upon
written  instructions,  received by the Company  within ten days of the intended
filing date of the registration statement,


                                                                             220
<PAGE>

to have included in such registration  statement the number of Conversion Shares
issued or issuable to them as such Holder shall so instruct.  The Company  shall
use its best  efforts  to  cause  such  registration  statement  to be  declared
effective  by the SEC as  promptly  as  possible  and to keep such  registration
statement effective at all times, as set forth above.

B. AMENDMENT OF SECURITIES PURCHASE AGREEMENT

1. Amendment of Article 1. Purchase and Sale of Securities

      Article 1 of the Securities  Purchase Agreement is hereby amended to as to
read as follows:

            1.  Purchase  and  Sale  of  Securities.   Upon  the  basis  of  the
      representations  and  warranties,  and subject to the terms and conditions
      set forth in this Agreement,  the Company  covenants and agrees to sell to
      the Purchaser on the Closing Date (as hereinafter  defined) 10 Units, each
      consisting  of:  (i) a  10%  Convertible  Subordinated  Debenture  in  the
      principal  amount of $25,000 United States  Dollars (each a  "Debenture");
      and (ii)  100,000  Warrants  to  purchase  a like  number of shares of its
      Common  Stock,  par value  $.001  per share  (the  "Common  Stock")  at an
      exercise  price of $.001 per share  (the  "Warrants").  The  Debenture  is
      convertible,  at any time, in accordance  with the terms and conditions of
      the Debenture, as amended the date hereof.

            The  Company  has  filed  a  registration  statement  under  the Act
      covering the Conversion  Shares.  The Company will use its best efforts to
      cause such registration  statement to be declared  effective by the SEC as
      promptly  as  practicable.  If a  Debenture  is not  converted,  it may be
      redeemed by the holder any time after  maturity  at 125% of the  principal
      amount of the Debenture plus all interest accrued thereon.

C. AMENDMENT OF REGISTRATION RIGHTS AGREEMENT

1. Elimination of Article 2(b)

      Paragraph (b) of Article 2 of the Registration Rights Agreement, is hereby
canceled,  terminated,  void and of no  further  force or effect to the end that
Tirex shall have no obligations or  liabilities  under the said Paragraph  2(b),
and the  Debentureholders do hereby remise,  release,  discharge,  indemnify and
hold  harmless  Tirex,  and  each  shareholder,  officer,  director,  affiliate,
associate,  agent, and employee of Tirex of and from manner of actions and cause
of action,  suits, debts, dues,  accounts,  bonds, wages,  benefits,  covenants,
contracts,  agreements,  judgments,  claims and demands  whatsoever in law or in
equity, and including without  limitation all such actions,  claims and demands,
etc.  arising out of, being based upon,  or being in any way  connected  with or
related to the said Paragraph 2(b).


                                                                             221
<PAGE>

D. FURTHER AMENDMENTS

      To the extent necessary to give effect to the amendments, contained herein
(the  "Amendments"),  to  the  Debenture,  Securities  Purchase  Agreement,  and
Registration  Rights  Agreement,  all remaining  provisions of such  agreements,
which may be inconsistent  with the Amendments,  are hereby deemed amended so as
to be consistent with the intents and purposes of, and are included  among,  the
Amendments.

E. NO OTHER AMENDMENTS

      Except  as  expressly  provided  in this  Amendment,  all of the terms and
conditions of the Debenture,  Securities  Purchase  Agreement,  and Registration
Rights Agreement remain in full force and effect.

F. TIMELY DELIVERY OF PROSPECTUS

      The Company  hereby  agrees that upon  effectiveness  of its  registration
statement  on Form SB-2  registering  the  resale of the shares  underlying  the
Debentures and Warrants,  it will promptly provide the  Debentureholders  and/or
their broker with a reasonably  sufficient quantity of Prospectus to effect such
resales.

G. COUNTERPARTS

      This Amendment may be executed in any number of  counterparts  and by each
party on a separate  counterpart,  each of which when so executed and  delivered
shall be an original, but all of which together shall constitute one Amendment.


                                                                             222
<PAGE>

      In Witness  Whereof,  the parties  hereto have caused this Amendment to be
executed the day and year first above written.


                                            THE TIREX CORPORATION

                                            By _________________________________
                                            ____________________________________
                                            ____________________________________


                                                                             223