H-2 production from a water-acetonitrile mixed solution containing iodide electron donor was investigated over dye-sensitized Pt/TiO2 photocatalysts under visible light irradiation. Merocyanine and coumarin dyes were utilized as photosensitizers. The rates of H-2 evolution, which decreased with the increasing water amount in the mixed solutions, were ascribed to the decrease in energy gap between the redox potential of I-3(-)/I- and the HOMO level of the dye. The observed high rate of H-2 production even in an aqueous solution over a coumarin-sensitized Pt/TiO2 photocatalyst shows that the dye has a sufficiently positive HOMO level relative to the oxidation potential of I-. (C) 2003 Elsevier B.V. All rights reserved.