Treatment of three wastewaters and two acid dye solutions by ozonation with varying ozone dose, initial pH, and initial dye concentration were examined. Ozonation of all the wastewaters were shown to be pseudo-first-order reaction with respect to color, chemical oxygen demand (COD) and dissolved organic carbon (DOC). Increasing ozone dose increased the removal of color, COD, and DOC, as well as decreased ozone consumption ratio, and increased apparent rate constants. Ozonation was more successful at decolorization than the removal of COD and DOC or augmentation of biodegradability. Increasing initial dye concentration increased the ozone consumption and decreased the rate constant. Color removal efficiencies for different ozone dose application were in the range of 60-91% for sample A (dye bath effluent), and 70-96% for sample B (plant effluent). For sample A, COD and DOC removal efficiencies at several ozone dose applications were between 13-22% and 8-13% respectively. For sample B, they were between 11-33% and 6-15% respectively. Increasing pH could expand the efficiency of ozone slightly. Ozone consumption per unit color, COD and DOC removal at any time was found to be almost the same whereas the applied ozone dose was different at a constant pH. Ozonation increased the biochemical oxygen demand (BOD5) of sample A and B 1.22 and 1.37 times by consuming 300 and 225 mg ozone, respectively. Ozonation of acid dyes was a pseudo-first order reaction with respect to dye. Increases in dye concentration raised specific ozone consumption. Specific ozone consumption for Acid Red 183 (AR-183) dye solution with 50 mg V of concentration rose from 0.32 to 0.72 mg-O-3 per mg dye decomposed as the dye concentration was increased to 500 mg l(-1).