The photoelectrochemical properties of titanium dioxide layers, prepared by thermal oxidation of titanium at 500-750 degrees C, were compared with those of layers of particulate (Degussa) P25, especially for oxidation of oxalic acid. The thermally formed oxide layers had rutile structures with a particle size of about 100 nm. Values of incident photon-to-current conversion efficiencies increased with rutile layer thickness and reached a maximum at about 1 mu m. Photocurrents for particulate TiO2 layers were about one order lower than those for thermal layers, due to the poor contact among individual particles, resulting in high electric resistance of the whole layer. The presence of oxalic acid had no effect on the photocurrent of thermal TiO2 layers, while in the case of porous particulate layers, the photocurrent increased strongly, due to oxalate adsorption and subsequent enhanced oxidation rate with photogenerated holes. For oxalic acid concentrations <= 10(-3) M, the photocurrent decayed due to mass transfer limitations, resulting in oxalate depletion in the porous particulate layer.