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Films of TiO2 dispersed or coated with platinum were deposited on glass and Pt-buffered polyamide substrates respectively by magnetron sputtering. The photocatalytic activity of the films was evaluated through the decomposition of acetic acid under UV irradiation. The Pt-dispersed TiO2 film of approximately 1.5 wt % platinum shows a maximum activity due to for the formation of anatase phase with a fine grain size. Platinum particles ∼20Å in thickness coated on anatase film greatly improves activity. The activity shows a steplike dependence of film thickness where the critical thickness varies between 150 and 200 nm depending on the deposition temperatures. The correlation between defects and activity was verified by measuring either the temperature dependence of electric resistance or the shift of binding energy from XPS.