A comprehensive method for separating proton exchange membrane fuel cell (PEMFC) cathode catalyst-layer and gas diffusion layer (GDL) mass-transport overpotentials was derived utilizing kinetic and ohmic analysis of high humidified H-2/O-2 and low humidified H-2/air polarization data. A hybridized method was applied accounting for electrochemical surface area, hydrogen crossover, and exchange current density. This method delineates separate cathode GDL and electrode mass-transport overpotentials as a function of current density and operating time. The methodology is systematic and generalized and can be applied to polarization data from any type of durability test. The derivation was applied to periodic polarization data from a steady-state 1050 h durability test and is shown to provide an accurate breakdown of the sources of performance losses. Increases in mass-transport overpotential for the cathode GDL and oxygen reduction reaction overpotential were predominantly offset by improvements in the mass-transport overpotential of the cathode catalyst layer and reductions in the high frequency resistance. Little increase in the GDL mass-transport overpotential was observed during the first similar to 500 h period, but a substantial increase was seen during the second similar to 500 h period. The mass-transport overpotential of the cathode catalyst layer was almost negligible at the end of similar to 1000 h of operation, suggesting little O-2 diffusion resistance through the ionomer and adjacent void volume. (C) 2010 The Electrochemical Society. [DOI:10.1149/1.3454740] All rights reserved.