Cobalt sulfides have been known for more than 30 years to be active toward oxygen reduction, and cobalt selenides have shown less activity. In this paper, a theoretical analysis is made of the four-electron reduction reaction of oxygen to water over the mixed anion and cation (202) surface of the pentlandite structure Co9Se8, one of several selenide phases. Reversible potentials for forming adsorbed reaction intermediates in acid are predicted using adsorption energies calculated with the Vienna ab initio simulation program (VASP) and the known bulk solution values together in a linear Gibbs energy relationship. Comparison with an earlier theoretical analysis of pentlandite structure Co9S8 shows that the overpotential is predicted to be larger for the selenide by around 0.22 V. Cobalt selenide electrodes of unspecified stoichiometry were prepared chemically on glassy carbon discs, and polarization curves were measured using rotating discs. When heat-treated at 900 °C, the onset potential for O2 reduction was found to be 0.5 V (normal hydrogen electrode, NHE), whereas electrodes not subject to heat-treatment were inactive. For Co3S4, onset potentials in the literature are ∼0.8 V (NHE), consistent with a ∼0.3 V higher measured overpotential for the selenide. The theoretical predictions for the pentlandite sulfide and selenide surfaces are in qualitative agreement.