The photochemical oxidation of ferrous ion in 0.8 N H2SO4 was investigated in the absence and presence of O2. The yields of H2 and Fe3+ were determined and the dependence of the quantum yield on light intensity, Fe2+ and Fe3+ concentration, and on the concentration of O2 was investigated. The post-irradiation effect due to H2O2 formation was followed. The inner filter effect due to the ferric ions formed in the reaction was taken into account. As a result, a mechanism accounting for the experimental facts could be derived. The excited state resulting on light absorption yields H atoms. H atoms oxidize ferrous ions in acid solution and also react with O2, if present. The competition between O2 and Fe2+ for the H atoms formed was investigated. It appears that the direct effect of O2 on the excited state is relatively small.