Cost and durability are central issues in the commercialization of proton-exchange membrane fuel cells. This study used complex accelerated-degradation protocols to diagnose the degradation modes of low-cost polytetrafluoroethylene-Nafion membrane electrode assemblies (MEAs). Acceleration protocols included open-circuit voltage, relative humidity cycling, and load cycling. Failure modes included measurements of cyclic voltammetry, linear sweep voltammetry, polarization curve, and AC impedance. The four modes (stages) of degradation were determined to be (i) catalyst aging, (ii) creep deformation, (iii) pinhole formation, and (iv) membrane failure. During catalyst aging, the maximum power density decreased by 0.117 mW cm(-2) cycle(-1). After the 280th cycle, creep deformation occurred, and the maximum power density decreased by 0.227 mW cm(-2) cycle(-1). Pinholes led to membrane failure and a final dramatic loss of performance (-0.453 mW cm(-2) cycle(-1)). Therefore, membrane failure is the major factor in the failure of MEAs. Copyright (C) 2010 John Wiley & Sons, Ltd.