The hydrogenation reaction of maleic acid, catalyzed by platinized platinum, was investigated in the presence and in the absence of gaseous hydrogen, and with/without an external control of platinum potential, in different acid supporting electrolytes. The intrinsic catalytic activity of platinum, used as a catalyst and as an electrode, was measured at different potentials. A promotion of the catalytic activity of platinum, induced by an external control of its potential, in the presence of gaseous hydrogen, was found only in a perchloric acid supporting electrolyte. No promotion effect was obtained in the presence of strongly adsorbed anions (HSO4-, Cl-). Moreover strongly adsorbed anions induce a decrease in the catalytic activity of platinum, whereas they do not affect the electrocatalytic activity of platinum catalyst-electrode. The observed anion effects on the hydrogenation of maleic acid were compared with those obtained in the electrocatalytic hydrogen oxidation (h.o.r.) and evolution (h.e.r.) reactions studied on a rotating disk platinum electrode. In electrocatalytic oxidation of hydrogen the current densities depend on the nature of the supporting electrolyte and decrease in the order, j(HClO4) > j(H2SO4) > j(HCl). However, the hydrogen evolution reaction, studied in underpotential and in low overpotential regions, does not depend on the nature of the supporting electrolyte. The ensemble of results is discussed in connection with the nature of active hydrogen species and/or active platinum sites involved in these reactions.