Phosphotungstic acid (PWA) was utilized as the reductive energy storage substance instead of conventional WO3 for the fabrication of an energy storage photocatalyst, which retains its reducing ability even after light irradiation. In the case where a PWA film was electrochemically charged at 20.8 V vs. Ag/AgCl for 1 h in 3 wt% aqueous NaCl, it showed longer self-discharge time than a WO3 film of the same thickness. Photoelectrochemical properties of the PWA and WO3 films coated with porous TiO2 were also studied. The PWA and WO3 films could be charged under UV light (1 mW/ cm(2)) for 1 h in 3 wt% aqueous NaCl. Although the amount of charge accumulated in the PWA-TiO2 film was smaller than that in the WO3-TiO2 film, the self-discharge time of the former was longer. Slower self-discharging process by O-2, which may be advantageous for some applications, of the former is responsible for these results. The PWA-TiO2 film showed relatively good durability after several cycles. (C) 2003 The Electrochemical Society.