The key phenomena in the electrolyte membrane for the adequate water management are water adsorption, water diffusion, electro-osmotic drag and water generation. The adsorption properties of water vapor on the membrane were quantified in this study as a first step of quantitative elucidation of these water transport phenomena. The relationship among the amount of adsorbed water on Flemion (R) membrane, partial pressure of water vapor and temperature was measured by using a micro balance and a thermohygrometer. The experimental values obtained in this study form sigmoid curves, and the adsorption model developed from finite layer B.E.T. equation can elucidate these experimental values. It was confirmed that the adsorption model can be applied also to Nafion (R) membrane and that applicable range of this model is from 10 to 80 degrees C. Moreover, it was found in this study that the amount of clustered water has close relation with the proton conductivity of the membrane. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.