Long-term (3422 h) operation of proton exchange membrane fuel cell in stationary conditions causes two regimes of degradation of the cathode catalytic layer. Firstly, from the beginning of life until the first membrane electrode assembly sampling, at t = 1163 h, fast degradation of the fresh Pt3Co/C nanoparticles is monitored; classical degradation mechanisms of Pt-based electrocatalysts occur, such as carbon corrosion, crystallite migration, dissolution of the less noble metal (Co), and 3D Ostwald ripening. A second degradation regime sets up from 1163 h to 3422 h, during which the changes in composition and morphology are slower. At the end of the ageing test, three distinct populations of Pt–Co/C nanoparticles coexist: (i) Pt–Co/C core–shell particles characterized by an alloyed (but depleted, compared to the fresh material) core surrounded by a 3–5 monolayer thick Pt-rich shell, (ii) Pt–Co/C “hollow” particles containing a central cavity surrounded by a Pt–Co shell containing limited amount of Co atoms distributed at the atomic scale and (iii) pure Pt/C “hollow” particles, from which Co dissolution has been completed.Experimental evidences are provided that the Pt-rich phase remains stable, and maintains constant ORR activity over more than 2000 h of operation in real PEMFC conditions.