Carbon-supported Pt–Co alloy catalysts with Co contents of 10–50 wt% have been synthesized by reducing aqueous Co²⁺ ions with borohydride or precipitating as hydroxide in the presence of a commercially available carbon-supported Pt catalyst followed by heat treatment in a 90% Ar–10% H₂ mixture at 900 °C for 1 h. X-Ray diffraction indicates the Pt–Co alloys thus obtained to have the ordered Pt₃Co or PtCo type structures or the disordered Pt type structure depending on the Co content. High resolution transmission electron microscopy (HRTEM) studies also confirm the formation of ordered structures. Structural analysis of the products after annealing at various temperatures 500 ≤ T ≤ 900 °C for 1 h suggests the ordering to be maximized for an annealing temperature of around 650 °C. Evaluation of the Pt–Co/C alloy catalysts for oxygen reduction in half cells employing KOH as the electrolyte and in proton exchange membrane fuel cells indicates that the alloys with the ordered Pt₃Co or PtCo type structures have higher catalytic activity with lower polarization losses and higher power densities than Pt or disordered Pt–Co alloys. With the ordered phases, the activity increases with the extent of ordering. The enhanced catalytic activity is explained based on optimal structural and electronic features consisting of optimum number of Pt and Co nearest neighbors, Pt–Pt distance, and d-electron density in Pt.