WO3 is a visible-light-responsive photocatalyst and an n-type semiconductor photocatalyst, but its photocatalytic activity toward organic substances is low. We utilized several transition metal ions as redox mediators in attempts to improve the photoactivity of WO3, and a drastic improvement was observed only when Cu2+ ions were utilized for WO3-catalyzed photodegradation of organic substances. More specifically, the CO2 generation rates for the photodegradation of formaldehyde and methanol in the presence of Cu2+ ions were about 1000 and 150 times as high, respectively, as the rates in the absence of Cu2+ ions. We confirmed that Cu2+ was reduced to Cu+ through the transfer of a photoexcited electron from WO3 and that Cu+ was easily re-oxidized to Cu2+ by oxygen in air. This redox cycle could assist in the reduction of oxygen in the photocatalytic reaction, and the reaction was particularly efficient when Cu2+ was introduced to the system as a CuCl2 solution. Photodegradation was achieved with a WO3 photoelectrode in a two-compartment cell in which the Cu2+ solution was separated from wastewater containing organic substances by an ion-exchangeable membrane, simulating practical use. The anodic photocurrent generated by the photodegradation of organic substances was observed at the WO3 photoelectrode without applying any external bias. (c) 2008 Elsevier B.V. All rights reserved.