Photocatalytic oxidation of water on TiO2 (rutile) powder proceeded with a fairly high efficiency (about 9%) when iron(III) ions were used as the electron acceptor. The reaction continued until all iron(III) ions added to the solution were reduced into iron(II) ions. This behavior was in marked contrast to other reversible photocatalytic reactions, whose reaction rates decelerate as the result of the back reactions of the products on the photocatalysts. The efficient oxidation of water in the presence of iron(III) ions was attributed to preferential adsorption of iron(III) ions on TiO2 over iron(II) ions, which enabled efficient oxidation of water, although this reaction was thermodynamically less favorable than oxidation of iron(II) ions. Furthermore, from the measurements of photocurrents at crystalline TiO2 electrodes, iron(III) ions were concluded to have a catalytic function for the oxidation of water on photoirradiated TiO2.