In situ internal reflectance infrared spectroscopy has been used to monitor the adsorption from aqueous solution and the photocatalysis of glyoxylic acid (HCO-COOH) on a thin TiO2 particle film deposited on a ZnSe single reflection prism. Glyoxylic acid was found to adsorb to TiO2 as the glyoxylate ion with bidentate coordination via the hydroxyls of the carboxylic acid group and of the hydrated aldehyde group. Near-UV irradiation of the adsorbed glyoxylate resulted in its conversion to adsorbed oxalate on the TiO2. A significant absorption at 878 cm(-1), which appeared under near-UV irradiation of the photocatalytic system, may be due to changes in the TiO2 surface structure or formation of surface peroxo species.