The electrocatalytic properties of nanocrystalline CoS₂ have been investigated for the oxygen reduction reaction (ORR) in 0.1 M HClO₄. CoS₂ with pyrite structure was prepared by hydrothermal synthesis and attached to a glassy carbon electrode from solution with a mixture of carbon and Nafion. The prepared CoS₂ electrode layers showed high activity toward the ORR and very good stability under oxygen reducing conditions. Selectivity of the ORR toward H₂O₂ was determined by rotating (ring) disk electrode measurements, and relatively high selectivity was obtained with up to 80% H₂O₂ formation around 0.4 V (vs Ag/AgCl), but this dropped to zero for potentials below 0.0 V. The amount of H₂O₂ produced between 0.6 and 0.0 V was dependent on the quality of the CoS₂ dispersion within the electrode layer, and decreasing CoS₂ particle size resulted in significant improvement in the ORR electrocatalytic activity, both by increasing the turnover frequency and through decreasing the selectivity toward H₂O₂ production.