Wastewater from textile processing plants can be highly coloured and difficult to decolourize. The present study describes the photo-catalytic decolourisation of an azo dye, namely Acid Orange 7 (AO7), in aqueous solution by combined UV and hydrogen peroxide. The reactor used in this study is a continuous laboratory-scale photo-reactor equipped with a low-pressure mercury lamp. Different initial dosages of hydrogen peroxide at variable initial concentrations of dye were used. The rate of colour removal was monitored spectrophotometrically at the visible maximum absorption wavelength. Decolourisation was complete in 15 min and follows apparent first-order kinetics. It has been found that the decolourisation rate increased up to an optimum value, beyond which the reagent exerted an inhibitory effect. Indeed, for each dye solution there is an optimum value for H2O2 initial dosage at which the pseudo-first order reaction rate constants have the maximum values. The apparent rate constant (k) values for each case have been determined. (C) 2004 Elsevier B.V All rights reserved.