The photocatalytic decolourisation and degradation of an azo dye reactive orange 4 (RO4) in aqueous solution with TiO2-P25 (Degussa) as photocatalyst in slurry form have been investigated using solarlight. There is a significant difference in adsorption of dye on TiO2 surface with the change in solution pH. The effect of various photocatalysts such as TiO2-P25, TiO2 (anatase), ZnO, US, Fe2O3\, SnO2 on the decolourisation and degradation has been studied. The order of reactivity of photocatalysts is TiO2-P25 > ZnO > TiO2 (anatase). US, Fe2O3 and SnO2 have negligible activity on RO4 decolourisation and degradation. The effects of various parameters such as catalyst loading, pH and initial concentration of the dye on decolourisation and degradation have been determined. The degradation was strongly enhanced in the presence of electron acceptors such as H2O2, (NH4)(2)S2O8 and KBrO3, ie effects of dye-assisting chemicals such as Na2CO3, NaCl have been carried out. Addition of these chemicals inhibits the removal rate. The photodecolourisation and degradation kinetics are discussed in terms of Langmuir-Hinshelwood kinetic model. (C) 2003 Elsevier B.V. All rights reserved.