The reduction of O-2 as catalyzed by the complex of Co(II) with the macrocyclic ligand C-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetrade (hmc) was examined using electrochemical techniques. The metastable (hmc)CoOO2+ complex is an intermediate in the catalytic cycle. It is in rapid equilibrium with Oz and (hmcCo2+. The (hmc)CoOO2+ complex is a strong oxidant that is rapidly reduced to (hmc)CoOOH2+ at the potential where (hmcCo3+ is reduced to (hmcCo2+ (0.38 V vs NHE). At more negative potentials (0.07 V) (hmc)CoOOH2+ is reduced irreversibly to H2O2 and (hmcCo2+ and the catalytic electroreduction of Oz to H2O2 ensues. The electroreduction of (coordinated) Oz to (coordinated) OOH- at potentials where uncoordinated O-2 is not reducible is attributed to stabilization of the reduced ligand by the cobalt center to which it is coordinated. Digital simulation was employed to obtain an estimate (0.99 V vs NHE) for the formal potential of the (hmc)CoOO2+/(hmc)CoOOH2+ couple at pH 1. The OOH- Ligand coordinated to (hmcCo3+ is electrooxidized to O-2 near 0.9 V vs NHE, but uncoordinated H2O2 is unreactive at this potential. The affinities of OO- and OOH- for (hmcCo3+ are comparable despite their highly disparate Bronsted basicities. Greater stabilization of the (hmc)Co00(2+) than of the (hmc)CoOOH2+ complex by LMCT is suggested to account for this result.