The ozonation of wastewater supplied from a treatment plant (Samples A and B) and dye-bath effluent (Sample C) from a dyeing and finishing mill and acid dye solutions in a semi-batch reactor has been examined to explore the impact of ozone dose, pH, and initial dye concentration. Results revealed that the apparent rate constants were raised with increases in applied ozone dose and pH, and decreases in initial dye concentration. While the color removal efficiencies of both wastewater Samples A and C for 15 min ozonation at high ozone dosage were 95 and 97%, respectively, these were 81 and 87%, respectively at low ozone dosage. The chemical oxygen demand (COD) and dissolved organic carbon (DOC) removal efficiencies at several ozone dose applications for a 15 min ozonation time were in the ranges of 15-46% and 10-20%, respectively for Sample A and 15-33% and 9-19% respectively for Sample C. Ozone consumption per unit color, COD and DOC removal at any time was found to be almost the same while the applied ozone dose was different. Ozonation could improve the BOD5 (biological oxygen demand) COD ratio of Sample A by 1.6 times with 300 mg dm(-3) ozone consumption. Ozonation of acid dyes was a pseudo-first order reaction with respect to dye. Increases in dye concentration increased specific ozone consumption. Specific ozone consumption for Acid Red 183 (AR-183) dye solution with a concentration of 50 mg dm(-3) rose from 0.32 to 0.72 mg-O-3 per mg dye decomposed as the dye concentration was increased to 500 mg dm(-3). (C) 2002 Society of Chemical Industry.