Direct water splitting into H-2 and O-2 over Pt-loaded semiconductor photocatalysts such as Pt-TiO2 was investigated. We found that addition of a small amount of iodide anion, I-, into the aqueous suspension of Pt-TiO2-anatase photocatalyst significantly improved the splitting into H, and 02 with a stoichiometric ratio. The iodide anion was adsorbed preferentially onto Pt co-catalyst as iodine atom, I. This iodine layer effectively suppressed the backward reaction of water formation from H-2 and O-2 to H2O over the Pt surface. (C) 2003 Elsevier Science B.V. All rights reserved.