Polymer electrolyte fuel cells (PEMFC) have emerged as portable power source for future transportation and home based systems. Over the past years perfluorosulfonated cation exchange membranes have been used as solid polymer electrolyte in fuel cells, however, humidity control and water management have remained major issues in the area of stack development. Local drying that results in the changes in the local conductivity behavior and physical failure of the membrane is one of the key issues to be addressed in detail. It is therefore necessary to estimate these changes precisely to evaluate the membrane behavior during the fuel cell operation. Positron annihilation spectroscopy offers a very sensitive probe to the determination of polymer free-volume as well as electronic state of the polymer. In the present study we studied Nation 117 with varying temperature and humidity using positron annihilation spectroscopy. The ortho-positronium lifetime (o-Ps, i.e., tau(3)) which gives the size of the free-volume holes inside the polymer was determined. It was found that the o-Ps lifetime increases for relative humidity between 40% and 60% and thereafter it decreases in the temperature range of 62-95 degrees C. When the inversion point at which the o-Ps lifetime is the maximum was plotted with actual vapor pressure, it was observed that it has a logarithmic behavior with the ordinate intercept of 1 ns. This indicates that at higher temperature the membrane has larger free-volume holes; and also at higher temperature a larger vapor pressure is essential to saturate the membrane. Above this point the clusters expand and the free-volume hole size decreases. This also indicates the optimum vapor in the cell to be maintained during the fuel cell operation. The studies also indicated that there is a correlation between o-Ps lifetime and the hydrogen crossover current density under the similar temperature and humidity condition. (C) 2010 Elsevier BM. All rights reserved.