An arbitrator exceeds his powers by deciding claims on grounds not pleaded, as the arbitrator's powers derive solely from the arbitration agreement and are limited to matters pleaded. However, where a defence constitutes a complete answer to claims (such as a compromise agreement), it would be artificial and unjust to disregard that defence merely because it was not pleaded specifically in answer to those particular claims, provided it was pleaded elsewhere and proved. In such circumstances, no gross irregularity within the meaning of section 33(1)(b) of the Arbitration Act 42 of 1965 occurs, as the parties were afforded a fair hearing. The determinative question is not technical compliance with pleading rules but whether the arbitrator misconceived the nature of the inquiry or denied the parties a fair hearing. Where the substance of a complete defence was pleaded, led in evidence, and established without prejudice to any party, the dismissal of claims on that basis does not constitute a reviewable irregularity merely because of the precise location of the pleading.