Electrochemical corrosion of carbon in a membrane electrode assembly (MEA) is a critical issue to be resolved to satisfy the durability targets for commercialization of proton exchange membrane (PEM) fuel cells. In order to understand the dominant mechanisms of MEA degradation by carbon corrosion, accelerated stress tests (ASTs), which consisted of holding at 1.3V for 50h. were conducted in a single cell, and the microstructural changes of the cathode catalyst layers (CCLs) and polarization losses were analyzed. The results of carbon corrosion were an increase in CCL porosity and 50% loss in the surface area of the Pt, leading to a 70% loss of the initial MEA performance at 0.6 V. Quantitative analyses of polarization losses for the MEA showed that oxygen diffusion was reduced significantly, while there were somewhat smaller increase of the kinetic and high frequency resistance (HFR) losses after carbon corrosion. Carbon oxide species that form on carbon particles during the carbon corrosion process make the surfaces of the CCLs more hydrophilic, and this is believed to diminish oxygen diffusion through the water-filled pores of the CCLs, leading to dramatic degradation of the fuel cell performance. However, another proposed mechanism of diffusion loss, which involves structural collapse and the loss of porosity in the CCLs due to carbon corrosion did not occur in the present study. These findings are anticipated to contribute to the development of more durable MEAs for PEM fuel cells. (C) 2012 Elsevier Ltd. All rights reserved.