This paper presents new results from XPS quantitative characterizations of cathode catalyst layers aged in a PEMFC with an anode operated under pure hydrogen and air and with 5 ppm CO contaminated hydrogen. Both oxygen rich and oxygen poor zones of the cathode catalyst layer were analyzed in order to show up heterogeneous degradation linked with gas distribution. The detailed chemical XPS analysis of the aged samples demonstrates in particular that in our operating conditions, the catalyst layer aging is mainly attributed to the oxidation of the carbon catalyst-support. A loss of the Nafion (R) ionomer in the cathode is also highlighted by XPS. Furthermore, the characterization of the cathodic catalyst layer chemical composition when CO is introduced in the anode side shows that the catalyst layer degradation is lower. These results are in agreement with the experimental-modeling work by Franco et al. [1] demonstrating that anodic CO contamination decreases the reverse proton pumping effect between the cathode and the anode and enhances the PEMFC durability. (C) 2010 Elsevier B.V. All rights reserved.