TiO2 P25 catalyst was used to study the photocatalytic oxidation of CO to CO2 at 288K. Two parameters, O2 and H2O were used to study its effect on the photocatalytic process. The dependency of the reaction rate on the CO concentration and water vapor was explained in terms of Langmuir-Hinshelwood mechanism. The presence of a high concentration of water vapor inhibits the CO photocatalytic oxidation at low oxygen concentration. We have noted an adsorption competition between CO and H2O on the TiO2 active sites.