Ozone, a stong oxidative agent, not only removes the color of the effluent but also reduces the organic pollution values. Numerous decolorisation studies on reactive dyeing effluents by ozonation has been made considering low fixation degrees of reactive dyes. Hydrolysis of reactive dyes had been reported in some studies in order to simulate actual reactive dyeing effluents. However, the hydrolysis of reactive dyes primarily effects the bonding capacity and its effects on decolorisation needs to be discussed. Also different hydrolysis procedures had been reported in some studies while no hydrolysis had been reported in some others. At this study, effects of various hydrolysis procedures using different temperatures, process time and alkalis on decolorisation and COD reduction of Cl Reactive Yellow 167 has been investigated. Results indicate neglicible effect of hydrolysis on decolorisation.