The durability of PEM fuel cells as a function of different humid conditions and MEA degradation were investigated by on/off cyclic operation. The maximum numbers of cycles determined by cycle at 10% of cell voltage loss were 1980, 1650, 1100, 520, and 320 cycles at relative humidity (RH) of 1000, 80, SO, 30, and 10%, respectively. The increases of ohmic resistance were observed at the high humid conditions of RH 100 and 80%. On the other hand, the charge transfer resistance increased at low humid conditions of RH 50, 30, and 10%. The losses of electrochemical surface area were obtained about 21% and 7% under RH 50 and 30% after the on/off cyclic operations, respectively. The thickness of electrode was reduced in the both anode and cathode layer, indicating that carbon corrosion occurred during on/off cycling. At cathode layer, the growth and migration platinum particles into polymer electrolyte were detected by XRD and TEM analyses. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.