Carbon supported platinum-ruthenium (Pt-Ru/C) nanoparticles are used as anode catalysts in proton exchange membrane fuel cells (PEMFCs) operated under reformate owing to their good carbon monoxide tolerance. The stability of these catalysts during fuel cell operation is still not well known. In this work, we have studied by transmission electron microscopy (TEM) the microstructural evolution of a membrane/electrode assembly after a 1000 h ageing test under reformate (26 ppm CO). The analyses clearly show dissolution of Ru from the Pt-Ru/C anode catalysts, its diffusion and precipitation within the anode micro-porous layer and within the membrane. The structure and the chemistry of the membrane precipitates were accurately analysed. The high resolution TEM images and EDS (Energy Dispersive X-Ray Spectroscopy) Pt, Ru elemental maps show that the largest precipitates display a singular flower shape consisting of a Pt-rich face-centred cubic (fcc) crystallographic structure core and Ru-rich hexagonal close-packed (hcp) crystallographic structure shell. These results suggest that within the membrane the Ru reduction is catalysed by Pt. Moreover, the localization of the precipitation band near the cathode seems to indicate that the Pt in the precipitates comes from the dissolution of cathodic Pt/C and that both Pt and Ru ions are reduced by the hydrogen crossover. (C) 2014 Elsevier B.V. All rights reserved.