Wastewater from textile processing plants can be highly colored and difficult to decolorize. During the past few years attention has been drawn to chemical techniques that could be used to textile wastewater decolorization. A crucial feature in designing such systems is the optimization of operating conditions. In the present study, advanced oxidation treatment, the UV/H2O2 process has been applied to decolorization of the azo dye C.I. Acid Orange 7 (AO7) in aqueous solution in a batch photo reactor. The effects of the reactor gap size and UV dosage on decolorization of dye have been investigated. The method of study involved monitoring the rate of dye solution decolorization during irradiation by a low-pressure mercury lamp and varying gap size and volume of the reactor. The rate of color removal was studied by measuring of the absorbance at characteristic wavelength. The gap size of the reactor was adjusted by different depths of the reactor. The results of this study showed that the removal efficiency of AO7 is optimal with 0.3 cm gap size and 83.33 W 1(-1) of UV dosage. (c) 2005 Elsevier B.V. All rights reserved.