Water is oxidized to molecular oxygen on a TiO2 photocatalyst using electron acceptors such as Ag+ and Fe3+ ions. However, there has been no information about whether this reaction occurs or not using molecular oxygen as the electron acceptor. We found by analysis of oxygen in the gas phase using O-18-enriched water that this reaction does in fact occur. The quantum efficiency was higher with rutile-form particles than with anatase-form particles. O-2 was photocatalytically produced using O atoms supplied solely from water even in the presence of O-2 in the system. These results are important for deepening the understanding of TiO2 photocatalysts because molecular oxygen is used most frequently as the electron acceptor and the intermediates of oxidation of water are involved in various kinds of photocatalytic reactions. (c) 2012 Elsevier B.V. All rights reserved.