The ozonation of wastewater containing azo dye in a semi-batch reactor has been studied. Results revealed that the rate of ozone transfer increased with increases in the initial dye concentration, the applied ozone dose and temperature. A model was developed to predict the enhancement factor of ozone mass transfer. This model enables the prediction of mass transfer coefficient of ozone from the following parameters: initial dye concentration, applied ozone dose, temperature and concentration of dissolved ozone in the organic-free water. This model was also valid for reactors of larger sizes. Result of kinetic studies showed that ozonation of the azo dye was a pseudo-first-order reaction with the applied ozone dose and temperature. However, the apparent rate constant declined logarithmically with increasing intial dye concentration. In addition, ozonation reduced chemical oxygen demand and enhanced the biodegradability of the wastewater. ⓒ 2001 Elsevier Science Ltd.