Observations of microstructural evolution by scanning and transmission electron microscopy in different regions of aged membrane/electrode assemblies (MEA) have shown that degradation is not always uniform through the MEA surface; after load cycling operation, the degradation is more severe in the region located near the air inlet compared to air outlet. After constant load operation the degradation appears more uniform. Two types of microstructural evolution have been observed. The first one consists in the modification of the cathode leading to nanoparticles larger than initially but still well dispersed. The second type of evolution ends up with big agglomerates formed by large Pt particles in the cathode, with also a noticeable degradation of the carbon support, both phenomena being always coupled with the precipitation of Pt particles inside the membrane. The first type of evolution results from the electrochemical Ostwald ripening mechanism and appears when the cathode potential remains low. In contrast, the second one appears when the cathode is locally exposed to a high interfacial potential resulting from the reverse-current mechanism. Hydrogen starvation induced by the load cycles and oxygen crossover that increases with membrane damage, are the two main factors responsible for this severe degradation mechanism. (C) 2012 Elsevier B.V. All rights reserved.