The influence of the membrane thickness on the performance and durability of 25 cm(2) membrane electrode assembly (MEA) toward dynamic aging test was investigated. The tested MEAs consist of chemically stabilized membranes (AQUIVION (TM)) with thicknesses of 30 and 50 mu m, electrocatalyst - 46 %Pt/C (Tanaka) with Pt loadings of 0.25 (anode), 0.45 mg cm(-2) (cathode) and gas diffusion layers 25 BC (SGL Group). The applied dynamic aging procedure is repetitive current cycling between 0.12 A cm(-2) for 40 s and 0.6 A cm(-2) for 20 s. The testing conditions were 80 degrees C, fully saturated hydrogen and air, total pressure of 2.5 atm abs. The aging procedure was regularly interrupted for evaluating the MEAs' "health" via electrochemical methods and mass spectrometry. The carbon support degradation as a function of the electrode potential, current cycling and supplied gas was studied. The effects of the Pt particles agglomeration and Pt physical loss in the active layer of the cathode on the MEAs performance degradation were individually assessed. The effect of the membrane thicknesses on the performance and durability of the PEFC was established. The reasons and stages of MEAs performance degradation were analyzed. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.