In this article, we focus on the understanding of the PtxCoy electrocatalysts degradation in polymer electrolyte fuel Cell (PEFC) environments. A multiscale atomistic/kinetic model is derived providing mechanistic insights on the impact of the nanostructure and operating conditions on PtxCoy nanoparticles durability. On the basis of ab initio (Al) data, we identify favorable pathways of the oxygen reduction reaction (ORR) on PtxCoy nanoparticles and of the competitive Pt-Co dissolution in acidic media. The derived Al kinetics is coupled to a description of the atomic reorganization at the nanoparticle level as a function of the cumulated Pt and Co mass losses. This nanoscale model is coupled with a transport microscale model of charges and O-2 through a PEFC cathode, and simulation sensitivity studies to operating, conditions and initial compositions/morphologies are performed and complimented by microstructural and electrochemical characterizations carried Out on aging direct liquid injection metallorganic chemical vapor deposition elaborated model electrodes detailed in our experimental companion paper. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3056048] All rights reserved.