MoO3 was characterized and compared with WO3 as an energy storage material for TiO2 photocatalysts. MoO3 exhibits larger charging and discharging capacities than does WO3 when electrochemically charged at -0.4 V versus Ag/AgCl (photopotential of TiO2). Self-discharging is also faster, reflecting greater ability for oxygen reduction. MoO3 combined with TiO2 is chargeable under UV light in a NaCl solution and in moisturized air. (C) 2004 Elsevier Ltd. All rights reserved.