O-3/UV oxidation of Cationic Red X-GRL was investigated in a semi-batch column reactor under various operating conditions. The absorption of ozone into aqueous solution and subsequent oxidation of Cationic Red X-GRL can be described by an irreversible second order reaction kinetic model. The relative contributions of direct ozone oxidation and (OH)-O-center dot-facilitated indirect oxidation of the dyestuff was quantified, and the overall reaction rate constant and the kinetic regime were determined by interpreting the experimental data with a newly derived kinetic model. A modified Arrhenius Equation as a function of T and pH was obtained with an activation energy of 15.31 kJ mol(-1) and exponential constant of 0.0093. The Hatta number of the reaction ranges from 0.041 to 0.085, indicating that the reaction occurs in the slow kinetic regime. A mechanistically sounder model was derived to describe the reaction kinetics, which takes into account mass transfer, ozone decomposition mechanisms, photolysis, and Cationic Red X-GRL degradation pathways. The model was able to adequately interpret the measured concentrations of Cationic Red X-GRL, dissolved ozone, off gas ozone, hydroxyl peroxide, and nitrate. Furthermore, a model-based sensitivity analysis of concentrations and ratios of O-3/UV system were performed with respect to various model parameters. (C) 2011 Elsevier B.V. All rights reserved.