The one-electron reduction of methyl viologen (MV2+) mediated by the carbon dioxide radical anion (CO2.- ) during photocatalytic reactions in a colloidal TiO2 aqueous solution (pH 2) has been investigated by time-resolved absorption spectroscopy. The formation of MV.+ generated from the one-electron reduction reaction with CO2.-, which is generated from the one-electron oxidation reactions with the photogenerated holes (h(+)), was directly observed. The spectral features of the photogenerated charge carriers and the kinetic analysis of the formation process of MV.+ revealed that the CO2.-, desorbed from the surface, reacts with MV2+ via a homogeneous electron-transfer process in the bulk solution.