The distribution of reaction products at the irradiated surface of a TIO2-ITO (indium-tin oxide) composite film was measured electrochemically using a carbon disk microelectrode located adjacent to the film surface (50-500 mu m). Under UV irradiation in 0.1 mM KCl aqueous solution, a sustained increase in the cathodic current at the microelectrode was observed in the vicinity of the TiO2 surface, whereas next to the ITO surface, the current was observed to flow in the opposite direction (anodic). The magnitude of the current depended on the distance between the film and microelectrode. The cathodic current response next to the TiO2 surface is due to the reduction of ClO- formed at the irradiated TiO2 surface, and the anodic response near the ITO surface is attributed to the oxidation of intermediate species formed by the reduction of the dissolved oxygen on the ITO surface. These results clearly show the formation of a "localized battery" at the irradiated TIO2-ITO film and also that the reaction products at the oxidation and reduction sites of the photocatalysts can be monitored separately using the present microelectrode system.