The photoreduction of Ag ion was studied on ZnO single-crystal surface: Zn polar surface (0 0 0 1) and O polar surface (0 0 0 (1) over bar). The single crystals were prepared by a hydrothermal process. Although Ag precipitate did not grow on a nonphotocatalytic crystal, such as Al2O3, under UV irradiation, Ag ion in aqueous solution was rapidly reduced on ZnO single-crystal surface under UV irradiation. We found that the reactivity of Zn polar surface was higher than that of O polar surface and that the Ag deposition modes differed. To clarify the reaction mechanism, we performed a chemical analysis of the solution and Ag precipitates on ZnO surfaces by ICP and measured the pH of the solution. The results show that zinc ions were dissolved into solution from ZnO single-crystal surface, i.e., by photolysis, but there was less photolysis at the Zn than O polar surface. (C) 2002 Elsevier Science B.V. All rights reserved.