Advanced Oxidation Processes (AOPs) have been used as emerging wastewater treatment technologies which can effectively handle various hazardous organics in wastewater and groundwater. This study examined the photodecolorization of azo dyes in aqueous solution by combined UV, individual and coupled photocatalysts. The model substrate employed in this work was C.I. Acid Yellow 23 (AY23) and the ZnO and ZnO/SnO2 were utilized as the photocatalysts. A 30W UV lamp was the source of UV-radiation in a batch reactor. Photodecolorization efficiency was small when the photolysis was carried out in the absence of ZnO and it was also negligible in the absence of UV light. The effects of various parameters such as catalysts loading, initial dye concentration, radiation intensity, pH, gap size and catalysts ratio on the dye removal have been investigated. The results showed that the removal efficiency of AY23 is optimal with 1.4 cm gap size, 40 mg L-1 of azo dye concentration and the catalysts amounts of ZnO and SnO2 were adjusted to be 700 mg L-1 and 200 mg L-1, respectively. Accordingly, it could be stated that the complete decolorization of color, after selecting desired operational parameters could be achieved in about 50 min. (C) 2010 Elsevier B.V. All rights reserved.