It is recognized that the behavior of water in the proton exchange membrane (PEM) is to be understood to optimize the performance of the proton exchange membrane fuel cell (PEMFC). It is considered by the present authors that a certain amount of structural water exists besides adsorbed water in the film of PEM. The structural water represents any chemically adsorbed water, crystal water and some chemical groups containing hydrogen which are strongly connected with the structure. Accordingly, the amount of structural water cannot be measured by the water adsorption method because the amount of structural water does not depend on the vapor pressure. The amount of adsorption together with the amount of structural water is measured in this study by using the tritiated water sorption method where the isotope exchange reaction between tritium in water vapor and hydrogen in structural water makes it possible to quantify the structural water. The observed values for the total amount of uptake water, which includes the structural water, is 0.45-0.57 (g H2O/cm(3) PEM) at the room temperature. This value is about 1.5 times larger than the amount reported so far from the water adsorption experiments. The permeation behavior of hydrogen, oxygen and helium through the film of PEM is also measured in this study and change of partial pressure of hydrogen or oxygen in PEMFC at stop of the fuel flows is estimated. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.