Active oxygen species formed on TiO2 film photocatalysts in air were examined by use of a chemiluminescence method. Under relatively strong UV light irradiation, 15 mW cm(-2), two types of active oxygen species formed on the TiO2 surface were observed after the interception of the excitation light, i.e., one with a shorter lifetime, ca. 3 s, and another with a longer lifetime, ca. 50 s, which may correspond to superoxide (O-2(-)). On the contrary, under relatively weak UV light irradiation, 1 mu W cm(-2), only the long-lived species, O-2(-), was observed. The concentration of O-2(-) on the TiO2 surface was saturated even under weak UV illumination, while the short-lived species was not. We suggest that O-2(-) is deactivated mainly by an electron transfer to surface defects or vacancy sites on the TiO2 surface. This deactivation reaction proceeds almost independently of coexisting active species.