Ozonation of three different simulated reactive dye-bath effluents containing aminochlorotriazine dyes and several dye assisting chemicals was investigated at semi-batch mode and four different ozone doses. Colour and acute toxicity measurements have been conducted to follow the fate of these dyes during ozonation. Experimental results have indicated that none of the dye-bath effluents was toxic prior to ozonation (as raw dye-bath wastewater). Colour removal was prompt and complete within the first minute of ozonation at a dose range of 7 mg - 29 mg (respective ozone consumption was around 2 mg - 16 mg). However, toxicity increased during the progress of decolouration for the dye-bath containing copper complex dye Blue HERD, and extensive ozonation periods (i.e. excessive ozone doses) were required for toxicity reduction. The abrupt increase in dye-bath toxicity during the preliminary stages of ozonation was attributed to the rapid release of complexed copper and subsequent formation of organic copper complex intermediates. Thereafter, toxicity values decreased speculatively as a consequence of the copper complex fragmentation.