It is well known that the membrane electrode assembly (ME) of proton exchange membrane fuel cells (PEMFCs) can undergo deterioration during long term operation of both the electrode materials and the membrane Hydrogen crossover i e the undesired diffusion of the gas from the anode to the cathode through the membrane has been ascribed as one of the main causes of deterioration of perfluorinated ionomer membranes normally employed in PEMFCs One of the effects of the hydrogen permeation across the membrane is the decrease of the cell s open circuit voltage (OCV) due to the reaction between the fuel and the oxidant at the cathode surface Such reaction can lead to the production of peroxide radicals causing the degradation of both the PEM and the catalyst layer Hydrogen crossover Increases when temperature pressure and humidity of the cell rise The hydrogen permeation rate through a very thin PEM is typically lower than 1 mA cm(-2) for a new MEA but it can exceed 10-20 mA cm(-2) after long term operation Various methods have been proposed to measure the rate of hydrogen crossover mainly based on electrochemical tests on a single FC with a flow of mitogen at the cathode so that the steady state current corresponds to the oxidation of crossed hydrogen Hydrogen crossover has been also determined indirectly by assuming that the changes in the OCV values are due to the passage of fuel from the anode to the cathode In this paper a simplified mathematical model for the direct determination of hydrogen crossover permeation rate is presented Such a model is based on analytical expressions of the polarization ti rms and it is employed to determine the hydrogen crossover rate The main results show that the hydrogen crossover current densities increased from 0 12 to 032 mA cm(-2) by decreasing the thickness of the membranes and increasing the operating cell temperature Moreover the hydrogen crossover determined for a fresh MEA was compared with that of a degraded one exposed to repetitive freezing/thawing cycles It was found that the hydrogen crossover for the degraded MEA was more than twice the value obtained with the fresh one at the same temperature (C) 2010 Elsevier BV All rights reserved