The objective of this study is to investigate membrane electrode assembly (MEA) failure modes under accelerated test conditions via reactant relative humidity (RH) cycling. In this study, the anode and cathode underwent RH cycling in an unsymmetrical manner and the cell was run at a minimal ''idle" current during endurance testing. The cell performance was monitored periodically and the degradation curve showed a difference in the anode and cathode induced RH cycling modes. Anode RH cycling had a more pronounced effect on MEA degradation than cathode RH cycling. Electrochemical diagnostic testing methods such as AC impedance and H-2 crossover measurements revealed the degradation phenomena in more detail. The fluoride release data of the anode RH cycling cell showed a sudden increase in fluoride rate within a short period of endurance testing. The infrared imaging results revealed thinning and hotspot pinholes in the membrane, and ionomer delamination from the PTFE reinforcement layer was identified by scanning electron microscopy. The anode RH cycling cell had a shorter lifespan than the cell with cathode RH cycling, highlighting the significance of anode humidification for MEA durability. (C) 2012 Elsevier B.V. All rights reserved.