The degradation mechanisms of nanostructured Pt3Co/C electrocatalysts aged in dry electrochemical environment using a Nafion (R) 115 membrane as polymer electrolyte were characterized by Identical Location Transmission Electron Microscopy, in conditions that perfectly mimic real PEMFC operation. The structural, morphological and compositional changes of the Pt3Co/C nanoparticles occurring during an accelerated stress test were bridged to changes of their intrinsic kinetics toward the oxygen reduction reaction in Nafion (R) 115 electrolyte, thanks to an ultramicroelectrode with cavity loaded with the catalyst. The unique setup used herein further enabled to compare the Nafion (R) environment with conventional liquid electrolyte in which accelerated stress tests are usually performed. Although the Pt3Co/C nanoparticles are modified upon aging at Nafion (R) interface, the degradation processes are milder and different than those observed in liquid electrolyte, mostly following the absence of liquid water and the lack of ion mobility within the Nafion (R) membrane. (C) 2014 Elsevier B.V. All rights reserved.