The photocatalytic degradation of an azo dye acid red 18 (AR18) using ZnO as a photocatalyst in aqueous solution has been investigated under UV irradiation. The blank experiment for either illuminated AR18 solution or the suspension containing ZnO and AR18 in the dark showed that both illumination and the catalyst were necessary for the destruction of AR18. The effects of operational parameters such as the amount of photocatalyst, dye concentration and initial pH have been examined. The study on the effect of oxidants such as (NH4)(2)S2O8, KBrO3 and H2O2 on the photooxidation of AR18 reveals that the addition of (NH4)(2)S2O8 and KBrO3 increases the dye removal whereas the addition of H2O2 decreases the photocatalytic degradation. The unusual decrease by the addition of H2O2 is due to its low adsorption on the ZnO surface. (c) 2007 Elsevier B.V. All rights reserved.