Catalysts for the electroreduction of Oz consisting of a cobalt porphyrin containing three Ru(NH3)(5)(2+) groups coordinated to cyanophenyl ligands pendant to the porphyrin ring have been synthesized. When adsorbed on graphite electrodes, the 3-cyanophenyl derivative acts as an electrocatalyst for the two-electron reduction of O-2 to H2O2 while the corresponding 4-cyanophenyl derivative catalyzes the four-electron reduction of O-2 to H2O.