Sequential FTIR-ATR spectra of an illuminated Degussa P-25 film deposited onto the ATR crystal were recorded to follow the photocatalytic oxidation of oxalic acid in acidic media. The results show that illumination ensues in the depletion of adsorbed oxalate from its equilibrium condition. The equilibrium spectral and thermodynamic data obtained previously from measurements in the dark were used to derive the time evolution, upon illumination, of the surface coverage of each of the three known surface complexes formed by oxalic acid adsorbed onto TiO2. The results demonstrate that the most stable species is also the most photo-labile one, and that the surface speciation is determined by a fast surface redistribution among the three species, without equilibration with the bulk solution. Thus, zero-order kinetics are observed at high degrees of coverage during the disappearance of the least stable species. At low concentrations, the disappearance of the most stable species is characterised by first-order kinetics. © 2005 Elsevier B.V. All rights reserved.