Mn-porphyrin supported on active carbon, which was activated by heat-treatment in Ar, electrochemically catalyzed reduction of O-2 to H2O2 by the H-2/O-2 fuel cell method. The electrocatalytic activities were strongly dependent on the heat-treatment temperatures. The maximum H2O2 concentration of 3.5 wt % with 47% current efficiency was obtained for the catalyst treated at 450 degrees C, and a TON (Mn) for the H2O2 formation was over 1000 h(-1).