Article 19
1. Within a period of time to be determined by the arbitral tribunal, the respondent shall communicate his statement of defence in writing to the claimant and to each of the arbitrators.
2. The statement of defence shall reply to the particulars (b), (c) and (d) of the statement of claim (article 18, para. 2). The respondent may annex to his statement the documents on which he relies for his defence or may add a reference to the documents or other evidence he will submit.
3. In his statement of defence, or at a later stage in the arbitral proceedings if the arbitral tribunal decides that the delay was justified under the circumstances, the respondent may make a counter-claim arising out of the same contract or rely on a claim arising out of the same contract for the purpose of a set-off.
4. The provisions of article (18), paragraph 2, shall apply to a counter-claim and a claim relied on for the purpose of a set-off.

Amendments to the Claim or Defence

Article 20
During the course of the arbitral proceedings either party may amend or supplement his claim or defence unless the arbitral tribunal considers it inappropriate to allow such amendments having regard to the delay in making it or prejudice to the other party or any other circumstances. However, a claim may not be amended in such a manner that the amended claim falls outside the scope of the arbitration clause or separate arbitration agreement.