Abstract Quantum yields of from 0.235 to 0.385 have been determined for the reduction of Ag+ by the irradiation of 365 nm light on a ZnO surface in aqueous solutions containing 5×10−5 to 1×10−3 M AgClO4. It is proposed that these results can be interpreted by a reaction scheme based on the assumption that some conduction-band electron produced in the ZnO by the irradiation transfers to the Ag+ adsorbed on the ZnO surface. The surface-energy barrier under the present experimental conditions was discussed with the aid of the electrochemical measurement of the ZnO sinter. The photoreduction of Ag+ is optically sensitized by the addition of xanthene dyes, such as Uranine and Rhodamine B. In this case, it is observed that the photoreduction of Ag+ is accompanied by the photolysis of dye molecules. The apparent quantum yields (the number of reacted molecules/the number of photons absorbed in the suspension) of the dye-sensitized photoreduction of Ag+ and the photolysis of dye molecules are 0.015 and 0.007 for Uranine, and 0.003 and 0.0003 for Rhodamine B, respectively.