In this work, we operated a 16-cell proton-exchange membrane fuel cell (PEMFC) stack based on a Pt/C anode and a Pt3Co/C cathode under H-2/air at constant current (j = 0.6 A cm(-2)), temperature (T = 70 degrees C), relative humidity (RH) (anode: 0% RH, purge mode; cathode: 65% RH at the inlet), and pressure (1.3/1.1 bars absolute at the anode/cathode, respectively) for 1124 h. The membrane electrode assemblies were characterized with physical [field-emission gun-scanning electron microscopy (FEG-SEM), high resolution transmission electron microscopy, X-ray diffraction], electrochemical (cyclic voltammetry), and chemical (inductively coupled plasma-atomic emission spectrometry and X-ray energy-dispersive spectroscopy) techniques after different life stages, with special emphasis on the cathode electrocatalyst: as received (0 h), conditioned (17 h), and after 504 or 1124 h of aging. At the micrometer scale, FEG-SEM images reveal a very subtle degradation of the catalytic layers (thickness and density of defects). At the nanometer scale, the cyclic voltammograms of the cathode electrocatalyst point toward fast leaching of Co and formation of a "Pt skeleton" structure at the topmost surface layer within the first hours of operation. When the proton-exchange membrane fuel cell is operated for longer durations, two phenomena increase the "Pt character" of the electrocatalyst surface: Co surface segregation/leaching and Ostwald ripening. Consequently, the Pt-skeleton particles slowly evolve toward "Pt-shell/Pt-Co alloy core" structures with depleted Co content and a Pt-enriched shell (in the order of 2 atomic monolayers after 1124 h of operation). (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3485104] All rights reserved.