H-2 production in a water-acetonitrile solution containing iodide as an electron donor as investigated over dye-sensitized Pt/TiO2 photocatalysts under visible light irradiation, as a potential water-splitting system based on an iodide redox mediator The rates of H-2 evolution decreased with increasing proportion of water in the solutions, because of a decrease in the energy gap between the redox potential of I-3(-)/I- and the highest occupied molecular orbital levels of the dyes, which decreases the efficiency of electron transfer from I- to dye.