Gas diffusion layer (GDL) is a porous medium placed between the flow field and the catalyst layer in a proton exchange membrane fuel cell (PEMFC), and experiences electrochemical aging and mechanical stresses during usage. In the present work carbon cloth and carbon paper, two commonly used GDLs in PEMFC, are electrochemically aged in a simulated PEMFC environment. The results indicate that carbon paper is less prone to oxidation when compared to carbon cloth, which can be attributed to higher degree of graphitization of carbon fibers in the paper. However, carbon paper suffers greater loss of structural stability due to the adverse effects of aging on the fiber matrix interface. Increased weakening of paper when compared to cloth, after electrochemical aging, results in higher residual strain when subjected to cyclic compression and an increased intrusion of paper into the flow field channel when compared to cloth GDL. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.