The removal of C.I. Acid Orange 7 (AO7) from aqueous solution under UV irradiation in the presence of ZnO nanopowder has been studied. The average crystallite size of ZnO powder was determined from XRD pattern using the Scherrer equation in the range of 33 nm. The experiments showed that ZnO nanopowder and UV light had a negligible effect when they were used on their own. The effects of some operational parameters such as pH, the amount of ZnO nanopowder and initial dye concentration were also examined. The photodegradation of AO7 was enhanced by the addition of proper amount of hydrogen peroxide, but it was inhibited by ethanol. From the inhibitive effect of ethanol, it was deducted that hydroxyl radicals played a significant role in the photodegradation of the dye. The kinetic of the removal of AO7 can be explained in terms of the Langmuir-Hinshelwood model. The values of the adsorption equilibrium constant, K-AO7, and the kinetic rate constant of surface reaction, kc, were 0.354 (mg l(-1))(-1) and 1.99 mg l(-1) min(-1) respectively. The electrical energy consumption per order of magnitude for photocatalytic degradation of AO7 was lower in the UV/ZnO/H2O2 process than that in the UV/ZnO process. Accordingly, it could be stated that the complete removal of color, after selecting desired operational parameters could be achieved in a relatively short time, about 60 min. (C) 2006 Elsevier B.V. All rights reserved.