Ozonation of the commercially important reactive dyes Procion yellow HE4R (Reactive Yellow 84) and Remazol Black SB (Reactive Black 5) in actual dye rinse waters and simulated exhausted dye bath liquors has been studied. The effect of common dye auxiliaries (NaCl and Na2CO3) as well as the initial dye concentration on advanced oxidation kinetics was investigated. Decolourization was complete within a few minutes of ozonation in all cases whereas UV254 (absorbance at 254 nm wavelength), COD and TOC removals required higher ozone doses. It was found that NaCl did not affect the ozonation process at the present reaction conditions, whereas the addition of Na2CO3 resulted in a significant inhibition in the abatement of all selected process parameters except decolourization. Increasing the initial dye concentration had an even more negative effect on decolourization, oxidation and mineralization kinetics. A simple kinetic model describing the effect of inhibitor concentrations on COD and/or colour removal rate constants has been proposed.