Photoformed hole and electron on TiO2 adsrobing NH3 are trapped, respectively, by a NH3 species adsorbed on TiO2 and Ti4+ to evolve a NH2 radical and a Ti3+ species. The photoinduced electron transfer takes place between Ti4+ and adsorbed NH3 species efficiently, resulting in the formation of a NH2 radical. The NH2 radical on TiO2 was stably present after the photo-irradiation was ceased. It is quenched rapidly just after introduction of NO leaving the Ti3+ species.