Photocurrents collected from an interface between polycrystalline anatase and an aqueous solution of oxalic acid were measured as a function of oxalic acid concentration and photon flux at 365 nm. According to a kinetic model previously developed, such data can be used as a diagnostic tool for distinguishing between direct oxidation involving valence band holes and an indirect process, involving (surface bound) OH radicals. In the case of oxalic acid studied here, from the linear relation between initial slopes of photocurrent versus concentration and photon flux, a direct process is indicated. (c) 2006 Elsevier Ltd. All rights reserved.