Start-up/shutdown under inappropriate conditions are known to damage proton exchange membrane fuel cells. Several measurement techniques were already used to characterize their effects and the degradations are mainly attributed to carbon oxidation: a decrease of the cathode active area is commonly observed after start-up/shut-down cycles. In order to achieve a better understanding of the conditions in which this phenomenon appears, a segmented cell was designed and internal currents were recorded during start-up under open circuit conditions. The results of this work allow estimating the carbon losses at a global, as well as at a local scale, as a function of the hydrogen flow rate. Local electrochemical impedance spectroscopy measurements carried out on damaged MEA show clearly that the segments that remain for the longest time in the presence of air (while the others are fed by hydrogen) are the most affected by carbon oxidation. (C) 2011 Elsevier B.V. All rights reserved.