This study bridges the structure/composition of Pt-Co/C nanoparticles with their surface reactivity and their electrocatalytic activity. We show that Pt3Co/C nanoparticles are not stable during PEMFC operation (H-2/air: j = 0.6Acm(-2), T = 70 degrees C) but suffer compositional changes at the nanoscale. In the first hours of operation, the dissolution of Co atoms at their surface yields to the formation of a Pt-enriched shell covering a Pt-Co alloy core ("Pt-skeleton") and increases the affinity of the surface to oxygenated and hydrogenated species. This structure does not ensure stability in PEMFC conditions but is rather a first step towards the formation of "Pt-shell/Pt-Co alloy core" structures with depleted Co content. In these operating conditions, the Pt-Co/C specific activity for the ORR varies linearly with the fraction of Co alloyed to Pt present in the core and is severely depreciated (ca. -50%) after 1124 h of operation. This is attributed to: (i) the decrease of both the strain and the ligand effect of Co atoms contained in the core (ii) the changes in the surface structure of the electrocatalyst (formation of a multilayer-thick Pt shell) and (iii) the relaxation of the Pt surface atoms. (C) 2010 Elsevier Ltd. All rights reserved.