The important roles of OH radicals for remote oxidation using TiO2 photocatalysts were evidenced by the in situ detection of OH radicals in the gas phase using the laser-induced fluorescence (LIF) technique. The appearance of OD-LIF intensities after the exposure of D2O vapors over TiO2 powders and the decrease of the time-resolved signals of OH-LIF intensities with increasing calcined temperatures of TiO2 powders suggested that the exchangeable water at the TiO2 surface is the origin of the diffused OH radicals.