During proton-exchange-membrane-fuel-cell degradation various changes occur in the membrane-electrode assembly, and consequently the performance decays. Therefore, the correlation between physical property changes and the performance decay is important. In this paper, we focus on this correlation and have developed a quantitative model to estimate the performance decay after degradation. Comparison of electrochemical surface area (ECSA) and polarization curves before and after potential cycling demonstrates that there was more performance decay than that estimated due only to ECSA decrease, especially at higher current densities. This large performance decay is estimated and quantitatively simulated to be due to the increase in oxygen transport resistance caused by the oxygen transport through the ionomer film coating the Pt/C particles in the cathode catalyst layer. It is shown that the data can be fit by assuming that the effective ionomer film area is proportional to ECSA. According to our results, the transport resistance increased significantly during potential cycling and more than half of the performance decay at high current densities originated from the increased transport overpotential. The effect of proton overpotential in the cathode catalyst layer was also studied with additional performance decay witnessed under low-humidity conditions. (C) 2012 Elsevier By. All rights reserved.