Textile reactive dyes are very recalcitrant to biodegradation, causing aesthetic, acute and chronic toxicity problems in receiving waters. In this study, we present the degradation of three different reactive dyes (Reactive Black 5, Reactive Blue 19 and Reactive Blue 21) by ozone, at two different pH conditions. The ozonation process achieved a very fast decoloration rate for the studied dyes. The influence of pH in the ozonation of these dyes was only significant for Reactive Black 5. The toxicity of dyes, after pre-treatment, was evaluated using Vibrio fisheri (microtox). There was little change in toxicity for Reactive Black 5, but there was a reduction for Reactive Blue 19. A significant increase in toxicity for Reactive Blue 21 was verified, caused by the increase in the bioavailability of th e cooper ion that was in the complexed form before the treatment with ozone.