This is a report on the production of O-2 and H-2 from photocatalytic and photochemical processes in the WO3-H2O-Ce-aq(4+)-hv system. The photoproduction of O-2 and H-2 was studied over the range of WO, concentrations from 2 to 8 g dm(-3), and conduction band electron scavenger concentrations 1-20mN4 Ce-aq(4+). Medium and high concentrations of the electron scavenger gave mainly O-2 as the main product. Dilute solutions of [Ce-aq(4+)] < 2mM initially produced dioxygen, and then hydrogen after an induction period of 3-4h. Yields of 140-250 mu mol O-2 h(-1) and 1-7 mu mol H-2 h(-1) were obtained and were found to depend on the physical properties and content of WO3, the concentration of the electron scavenger, illumination period and wavelength, and the radiation geometry. The photoactivity of the suspension was correlated to the level of crystallinity of WO3 powders. The studied system utilizes WO3 to accomplish the initial light absorption, charge separation, and production of O-2 and H+ from the interaction of water molecules with photogenerated WO3 valence band holes, in the presence of Ce-aq(4+) species as a scavenger of conduction band electrons. This is followed by the evolution of H, from a homogeneous photochemical reduction of H+ and/or H2O by photoexcited Ce-aq(3+) formed from the earlier reduction of Ce-aq(4+). The obtained results show that, a a with an appropriate design, tungsten trioxide is a promising material that can be used as a photoactive component in energy conversion systems or in environmental photocatalysis, using artificial or solar light.