Oxalic acid oxidation in aqueous solutions of different compositions (different anions and pH), with a platinum catalyst-electrode, was studied. Different experimental methods were compared: the presence or absence of gaseous oxygen, external potential control or spontaneous establishment of the potential in the presence of the reactants. The intrinsic catalytic activity of platinum was determined by analytical techniques. The presence of strongly adsorbed anions in the solution induces a decrease of the catalytic activity of platinum. The weak 'anion-like' adsorption of oxalic acid, occurring in competition with the adsorption of different anions, can account for this result. Undissociated oxalic acid is more reactive than oxalate and dioxalate anions. The activity of platinum measured at a given potential, is higher in catalytic than in electrocatalytic oxidation of oxalic acid, suggesting that different adsorbed active oxygen species are involved in these two oxidation processes.