Abstract The complex of Co(III) with the macrocyclic ligand ph-tim (ph-tim = 2,3,9,10-tetraphenyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) adsorbs irreversibly on roughly polished pyrolitic graphite electrodes. The adsorbed complex exhibits two, reversible, one-electron voltammetric waves corresponding to Co(III)/Co(III) and Co(II)/Co(I) redox couples located at 0.32 V and −0.07 V, respectively. The adsorbed Co(II) complex generated at 0.32 V catalyzes the electroreduction of O 2 to H 2O 2 in a separate voltammetric step at more negative potentials which vary with pH. Catalytic plateau currents for the reduction of O 2 at rotating pyrolytic graphite disk electrodes coated with [(ph-tim)Co] ads 3+ ar elimited by the rate of coordination of O 2 to the reduced catalyst, [(ph-tim)Co]] ads 2+. An upper limit of 150 M −1 was placed on the equilibrium constant for the binding of O 2 to [(ph-tim)Co] ads 2+. A catalytic mechanism compatible with the results is proposed and discussed.