The effect of moisture in the reactant gas on CO oxidation over Au/TiO2 has been quantitatively investigated over a wide range of concentrations from 0.1 to 6,000 ppm. Moisture enhances the reaction by more than 10 times up to 200 ppm H2O, while further increase in the moisture content suppresses the reaction. The apparent activation energy is, however, independent of the amount of moisture. The amount of moisture adsorbed on the catalyst influences the activity rather than the moisture content in the gas phase, which suggests that the low-temperature CO oxidation over the gold catalyst involves water-derived species on the catalyst surface.