Unlike most simple, monomeric cobalt porphyrins, the cobalt tetramethylporphyrin identified in the title catalyzes the electroreduction of O-2 to H2O instead of H2O2. On graphite electrodes, coatings of the porphyrin prepared by irreversible adsorption from chloroform solutions catalyze the electroreduction of O-2 in aqueous HClO4 at unusually positive potentials. Both the title porphyrin and the corresponding unsubstituted cobalt porphine are argued to derive their activity as four-electron reduction catalysts from their propensity to form dimers (or higher aggregates) in solution and, presumptively, on the surface of graphite electrodes.