A novel class of Pt–Co secondary solid solution catalysts with long-range ordered intermetallic CoPt₃ as the solvent and Co as the solute for the oxygen reduction reaction (ORR) is proposed. The catalysts exhibit a volcano-type dependence on Co solid solubility in ORR activity and the optimum catalyst with 10% Co solid solubility shows remarkably enhanced catalytic activity and durability, which can be ascribed to the unique electronic structure of secondary solid solution catalysts.