Ozonation of Cationic Red X-GRL was investigated in a semi-batch column reactor under various operating conditions such as gas flow rate QG, temperature T, initial concentration CD,0, and pH. The relative contributions of ozone direct oxidation and OH-facilitated indirect oxidation of the dyestuff were quantified, and the overall rate constant kT 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 γ of indirect oxidation rate constant kR to kT 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 CD, dissolved ozone CA, ozone in off gas CA,G, and nitrate. Furthermore, a model-based sensitivity analysis of CD/CD,0, CA, and CA,G was performed with respect to various model parameters.