Platinum-iridium (Pt-Ir) alloy electrocatalysts for polymer electrolyte fuel cell (PEFC) cathodes were prepared, and their physical and electrochemical properties were examined. A variety of unsupported Pt-Ir alloy catalysts was prepared in hot KOH solution, and the obtained Pt-Ir alloy catalysts were aggregates of fine particles < 100 nm; the structure of the alloy was a uniform solid solution phase between Pt and Ir. Evaluation of the activity for the oxygen reduction reaction (ORR) was performed by both the rotating disk electrode (RDE) method in 0.1 M HClO4 solution and determination of the specific activity during PEFC single-cell operations. The ORR kinetic analysis by the RDE showed that alloying Pt with Ir, especially with 5-20 wt % Ir, enhanced the catalytic activity for the ORR by a factor of more than 1.5 at 0.8 V vs RHE. The relationship between the specific current density and Ir content under PEFC operating conditions also showed enhanced ORR activity at 5-20 wt % Ir, which was consistent with the RDE results. (c) 2005 The Electrochemical Society.