We present a set of XPS (X-ray photoemission spectroscopy) measurements and detailed data-analysis of electrodes for polymer electrolyte membrane fuel cell (PEMFC) based on carbon supported catalysts (Pt and Pt-Co) subjected to a step-like potential cycling (accelerated degradation test). The results of the measurements complete and corroborate parallel TEM (transmission electron microscopy), XRD (X-ray diffraction) and XAS (X-ray absorption spectroscopy) results pointing at a modification of the nanostructure of the catalyst grains. The surface sensitivity of the XPS measurements is used to study the modification of the reactive surface structure of the nanoparticles and the interplay of reactivity and dissolution/precipitation/agglomeration of metal clusters during the catalyst work. The combination of XPS results with XRD, XAS and TEM ones allowed us to obtain a model for the structural changes in Pt-Co nanoparticles occurring during operation. The main structural modifications, in which predominantly Co atoms are involved, occur during the first hours of catalyst operation changing the stoichiometry and degree of disorder in external particles shell. In the last stages of the degradation process, core shell particles structure, with ordered Pt4Co core and Pt-rich shell (and finally Pt-skin), is formed possibly explaining higher Pt-Co catalyst stability (structural and electrochemical) than pure Pt catalyst. (C) 2014 Elsevier B.V. All rights reserved.