Using density functional theory (DFT) calculations, we revisited a classical problem of uranyl(VI) oxalate photochemical decomposition. Photoreactivities of uranyl(VI) oxalate complexes are found to correlate largely with ligand-structural arrangements. Importantly, the intramolecular photochemical reaction is inhibited when oxalate is bound to uranium exclusively in chelate binding mode. Previously proposed mechanisms involving a UO₂(C₂O₄)₂²⁻ (1 : 2) complex as the main photoreactive species are thus unlikely to apply, because the two oxalic acids are bound to uranium in a chelating binding mode. Our DFT results suggest that the relevant photoreactive species are UO₂(C₂O₄)₃⁴⁻ (1 : 3) and (UO₂)₂(C₂O₄)₅⁶⁻ (2 : 5) complexes binding uranium in an unidentate fashion. These species go through decarboxylation upon excitation to the triplet state, which ensues the release of CO₂ and reduction of U(VI) to U(V). The calculations also suggest an alternative intermolecular pathway at low pH via an electron transfer between the excited state *UO₂²⁺ and hydrogen oxalate (HC₂O₄⁻) which eventually leads to the production of CO and OH⁻ with no net reduction of U(VI). The calculated results are consistent with previous experimental findings that CO is only detected at low pH while U(IV) is detected only at high pH.