The role of adsorbed water in photocatalytic processes on TiO 2 surfaces has been addressed using ATR-IR spectroscopy. Changes in the IR spectra of thin films of Degussa P25 TiO 2 photocatalyst have been observed in the presence of adsorbed water and under near-UV light. Removal of adsorbed water results in the appearance of absorption peaks at 828, 745, and 685 cm -1 attributed to TiO 2 surface phonon modes. UV irradiation of the dry TiO 2 results in a broad IR absorption which toward lower wavenumbers increases in absorbance, reaches a peak at 880 cm -1 , and then drops sharply to a minimum absorption at about 750 cm -1 . From observations of the behavior of P25 thin films under a drying regime and comparisons with transient IR studies, the broad absorption generated by the UV light has been assigned to excitation of shallow trap electrons to the conduction band with an excitation energy of ∼0.1 eV.