Ozonation of Cationic Red X-GRL was investigated in a semi-batch column reactor under various operating conditions such as gas flow rate Q(G), temperature T. initial concentration C-D,C-0, and pH. The relative contributions of ozone direct oxidation and (OH)-O-center dot-facilitated indirect oxidation of the dyestuff were quantified, and the overall rate constant k(T) and the kinetic regime of the reaction were determined by interpreting the experimental data with a newly derived kinetic model. The Hatta number of the reaction was found between 0.053 and 0.080, indicating that the reaction occurred in the liquid bulk, i.e. the slow kinetic regime. The ratio gamma of indirect oxidation rate constant k(R) to k(T) decreased from 11.50% at pH 9.24 to 2.47% at pH 3.15. A mechanistically sounder model was derived to describe the reaction kinetics, which takes into account mechanisms of ozone decomposition and dyestuff degradation, and gas-liquid mass transfer. Good agreements were obtained between the experimental and calculated concentrations of Cationic Red X-GRL C-D, dissolved ozone C-A, ozone in off gas C-A,C-G, and nitrate. Furthermore, a model-based sensitivity analysis of C-D/C-D,C-0, C-A, and C-A,C-G was performed with respect to various model parameters. (C) 2011 Elsevier B.V. All rights reserved.