The behaviours of various semiconductor powders (Ti02, Fe203, ZnO, ZrOz , SbzO4, CeOz and WO,) dispersed in oxalic acid solutions were examined either in the dark or under illumination at wavelengths shorter or longer than 400 nm in the presence of oxygen. All the specimens except Sb204 and W03 were degraded in the dark, ZnO being the most unstable. UV illumination increased the oxidation of oxalic acid in the presence of TiOz, ZrOz and CeOz, and the quantum yields were determined. Visible light enhanced the conversion in the presence of 'I'iOz, Fe20, and W03. Despite its weak absorption, TiOz was the most reactive, and the two Fe,O, samples exhibited markedly different activities. Doping of Ti02 with. Cr3+ ions (0.86 at.%) substantially decreased its activity in both spectral regions, probably because of increased electron-hole recombination. It is concluded from these results that the surface properties and the presence of recombination centres, which depend on the preparation method, are very important factors in determining the activity of a given oxide. The results of a more detailed study of the effects of oxygen, oxalic acid concentration, pH and temperature performed using the TiO2 sample provided evidence for the photocatalytic character of the oxidation induced by UV light, and a mechanism involving an attack on the C-C bond of adsorbed H&O, ions by atomic oxygen species activated by photoproduced holes is tentatively suggested.