The purpose of this study was to investigate the catalytic role of granular activated carbon (GAC) on the degradation and mineralization of two reactive azo dyes (C.I. Reactive Red 194 (RR194) and C.I. Reactive Yellow 145 (RY145) in aqueous solution by treatment with ozone. Decolourization of the aqueous solution, disappearance of the parent compound, COD and TOC removal were the main parameters investigated for this purpose. Formation of inorganic ions Cl-, SO42-, NH4+, NO3- and pH changes were also followed throughout ozonation of various systems. It was determined that ozone by itself is strong enough to decolorize these aqueous solutions contaminated by azo dyes and almost all COD disappeared by these treatments but efficient mineralization had not been achieved. The extent of TOC removal was about 25% when no granular activated carbon (GAC) had been used and this value was doubled in the presence of GAC. While the reactive hydroxyl radicals were formed on the surface of GAC, it was deduced that the main reactions of the organic species with these reactive radicals took place in the bulk of the solution. One other observation was that the mass ratio of GAC to organic substrate in the aqueous solution should have an optimum value for effective degradation of the organic substrates; excess GAC inhibits degradation reactions probably due to the adsorption of the organic species on the surface of GAC. (C) 2006 Elsevier Ltd. All rights reserved.