The reaction kinetics of electrons photoexcited in a Pt/TiO2 catalyst were observed by time-resolved infrared absorption in the presence of methanol-water vapor mixtures. The recombinative decay of the electrons was completed in microseconds on the catalyst exposed to a vacuum. The decay was suspended in the catalyst exposed to pure methanol vapor due to an effective hole-consuming reaction by adsorbed methoxy species. The suspension was released as partial water pressure was increased, indicating that the holes reacted with the methoxy species without the aid of adsorbed water or hydroxyl species. The excess electrons remaining in the catalyst were consumed by adsorbed water in a time domain of 0.1 s. The water-induced promotion of the methanol oxidation reaction described in the literature was interpreted with these results.