Maleic acid was oxidized on a platinized-platinum catalyst in a pressurized electrochemical reactor, at temperatures higher than 343 K, in a potential range where an active adsorbed intermediate is formed. In the presence of O-2, when potential was either spontaneously established or imposed by a potentiostat, similar activities and similar evolution of activities versus potential values were obtained. These results could imply that the same mechanism occurs, involving gaseous oxygen as oxygen source. Lower activities were obtained in electrocatalytic oxidation, in the absence of gaseous oxygen, involving a different mechanism.