Photo decolourisation of various reactive textile dyes such as Procion Brilliant Yellow M-4G (PBY), Procion Brilliant Magenta M-B(PBM) and Pr cion Brilliant Orange M-2R(PBO) have been studied using ZnO as photocatalyst. Complete decolourisation (100%) of dyes by sunlight was obtained when ZnO was used as a catalyst. Decolourisation of dyes by sunlight was found to be more effective as compared to UV light. This paper also discusses the effect of pH. H2O2 concentration, dye concentration and catalyst weight on the decolourisation of dyes. The decolourisation was not affected by changes in pH. The degradation kinetics involving Langmuir-Hinshelwood model for PBY has been studied in detail.