In regard to degradation phenomena in a polymer electrolyte fuel cell (PEFC), there is little information on voltage loss due to the excess accumulation of liquid water blocking pores for the gas supply in the cathode (flooding) observed in a long-term operation over 5000-10,000 h. In this study, modeling of this degradation phenomenon was attempted using ozone treatment of the cathode catalyst layer (CL). The hydrophilicity of the layer was increased by the treatment, which was attributed to oxidation of the carbon surface of the catalyst (Pt-loaded carbon black, Pt/C) exposed to the gas phase in the CL. The flooding was observed at the PEFC with the cathode CL formed using 10 wt % Pt/C and treated with ozone for 10 h. and the extent of the flooding was enlarged by the ozone treatment of 15 h. The flooding was caused by the 5 h ozone treatment when the Pt content was 20 or 50 wt %. These results indicated that modeling of the long-term degradation was achieved by the short-term ozone treatment, and that the excessive hydrophilicity of the carbon surface of Pt/C was one of the highly potent candidates for causes of the degradation. (C) 2009 The Electrochemical Society.