A 2520 h lifetime test of a single proton exchange membrane (PEM) fuel cell under low humidification (65.8% relative humidity for both anode and cathode feed stream) at a low current density of 200 or 300 mA cm^{â 2} was conducted. H2 crossover rate through electrolyte membrane and open circuit voltage of the cell with time were measured to understand the membrane degradation in the membrane electrode assembly (MEA) during the life test. The results show that low humidification of feed stream accelerates membrane physical degradation, and results in membrane pin-holes and reactant gas crossover. A novel sub-cell approach coupled with SEM and TEM were employed as post mortem analyses. The performance of the sub-cell at H2 inlet region is unacceptably lower than that at the H2 outlet region. SEM images of the used MEA in cross-section apparently revealed that membrane thinning occurred mainly at the H2 inlet region. TEM analysis of the electrocatalysts in the pre- and post-test cell demonstrated that the agglomeration of the Pt particles took place both at the anode and the cathode. Therefore, the non-uniform degradation of electrolyte membrane and the agglomeration of Pt catalysts are concurrently responsible for the degraded performance of the PEM fuel cell.