This paper provides further insights into the degradation mechanisms of nanometer-sized Pt3Co/C particles under various proton-exchange membrane fuel cell (PEMFC) operating conditions. We confirm that Co atoms are continuously depleted from the mother Pt3Co/C electrocatalyst because they can diffuse from the bulk to the surface of the material. The structure of the Pt-Co/C nanoparticles in the long-term is determined by a balance between Co surface segregation and formation of oxygenated species from water splitting. When the PEMFC is operated at high current density (low cathode potential, below the onset of surface oxide formation from water), a steady-state is reached between the rate of Co dissolution at the surface and Co surface segregation. Consequently. Co and Pt atoms remain homogeneously distributed within the Pt-Co/C particles and the thickness of the Pt-shell is maintained to a small value not detectable by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. When the PEMFC is operated at low current density (high cathode potential), the formation of surface oxides from water and the resulting "place-exchange" mechanism enhance the rate of diffusion of Co atoms to the surface. Consequently, the fresh Pt3Co/C particles form core/shell particles with thick Pt-shells and Co content < 5 at% and, ultimately, "hollow" Pt nanoparticles (Kirkendall effect). To the best of our knowledge, this is the first report on the formation of "hollow" Pt particles in a PEMFC. (C) 2011 Elsevier Ltd. All rights reserved.