The ozonation kinetics of an azo dye, namely, Acid Red-151 was investigated. Effects of pH, initial ozone, and initial dye concentrations on the ozonation of the dye were studied. The reaction orders were found to be one with respect to both ozone and dye concentrations for the studied pH range of 2.5-10. The reaction rate constants were obtained as 125.8, 95.4, 87.9, and 129.9 L mmol(-1) min(-1) at the pH values of 2.5, 5, 7, and 10, respectively. The initial ozonation rate decreased with the increasing pH front 2.5 to 7. A further increase of pH to 10, yielded higher initial rates compared to the rates at pH = 5 and 7. However, increasing the initial dye and ozone concentrations increased the initial ozonation rate of the dye. Depending upon the initial conditions, up to 99% decolorization was achieved by ozonation in the present system. Also, chemical oxygen demand (COD) was partially reduced by ozone; the highest COD reduction being 58%, occurred at pH = 2.5, initial ozone and dye concentrations being 0.0360 and 0.0440 mmol/L, respectively.