Surface-enhanced IR absorption spectroscopy (SEIRAS) was applied to in-situ observation of surface products during the photooxidation of gas-phase n-decane on TiO2 films coated with an island Au film The bands assignable to C=O stretching. O-H stretching, and H-O-H bending were observed with good intensity, indicating the formation of ketone(s) and water on the surface. Ketone as well as water was produced in the dark when a Au/TiO2 sample was illuminated in advance in O-2 and then exposed to n-decane. This result clearly indicates that oxygen species active for n-decane oxidation remain in the dark after stopping UV irradiation. The formation of ketone and water declined with increasing time lapse of n-decane introduction after illumination, from which the lifetime of the active oxygen species was estimated to be at least 1 min. The active oxygen species formed from O-2 on illuminated Au/TiO2 were concluded to be O- and O-3(-), judging from the results of ESR and oxygen-isotope tracing measurements so far reported.