Improving the performance of and popularizing the polymer electrolyte fuel cell (PEFC) requires a highly active and cost-effective catalyst for the electrode reactions, especially for cathodic oxygen reduction, which is the main cause of voltage loss due to the low reactivity. Conventional activated carbon loaded with dispersed Pt has been found to possess the potential for this; however, slow transfer of oxygen and protons in the pores, whose mean pore diameter is usually below 4 nm, hinders the electrode reaction. In the present study, the catalyst layer of the PEFC electrode was developed using activated carbon with trifluoromethanesulfonic acid adsorbed in the pores, and its activity for cathodic oxygen reduction was investigated. We found that the activity of the catalyst layer significantly increased, approximately an eightfold increase at most, compared to a conventional catalyst layer formed from carbon black with dispersed Pt. Regarding the electrode potential, 100-150 mV improvements were attained. The enhancement was attributed to fast transfer of oxygen and protons in the pores containing the acid, leading to effective utilization of Pt particles inside the pores and high performance of the PEFC. (C) 2004 The Electrochemical Society.