It has been demonstrated that cerium dioxide is a potential photocatalyst that can be used to decompose water to produce oxygen in aqueous suspension containing an electron acceptor, and the optimum parameters for the reaction have been investigated. The O-2 yield strongly depended on the duration of irradiation, CeO2 concentration, concentration of the electron acceptor, and pH of the suspension. The optimum photoproduction for O-2 was obtained under the following operating conditions: Illumination time: > 10 h, CeO2 concentration: 2-5 g dm(-3). [Ce4+]: 4-5 mM, pH < 3, and illumination wavelength < 420 nm. During long-term runs, CeO2 suspensions showed a satisfactory photostability and activity even after 400 h of illumination. The obtained data show that, with an appropriate design, cerium dioxide is a promising material that can be used as a photoactive component in photocatalytic reactions. The studied system utilizes CeO2 to accomplish the initial light absorption, charge separation, and O-2 evolution from the interaction of water molecules with holes photogenerated in the CeO2 valence band, in the presence of Ce-aq(4+) or Fe-aq(3+) species as an acceptor of conduction band electrons.