We examined the effect of polytetrafluoroethylene (PTFE) content and annealing temperature on oxygen reduction reaction (ORR) in phosphoric acid for a series of Pt/ carbon nanotube (CNT) electrodes. When PTFE is added, PTFE particles cover the electrodes, influencing the active Pt surface area, the adsorption of phosphate species on the Pt surface and mass transport of reactants to the surface. The performance of the Pt/ CNT showed maximum at 20 and 25% PTFE content for annealing temperatures of 337 and 350 S C, respectively. Performance was further improved by mixing Pt/ CNT with Pt/ C. This may be explained by improved accessibility of reactants to/ from the interface. (c) 2007 The Electrochemical Society.