Adsorption isotherm of water vapor on a sulfonated polyethersulfone (S-PES) membrane in the temperature range from 10 to 50, C is measured in the present study. The sigmoid curve isotherm that we proposed for the adsorption of water vapor on Flemion(R) and Nafion(R) is also applied to that of S-PES similarly. The obtained adsorption isotherm shows that varieties of the amount of water adsorbed in the S-PES with temperature is smaller than those of Flemion(R) and Nafion(R). Thus, varieties of the proton conductivity of the S-PES membrane with temperature is larger than that of Flemion(R) and Nafion(R), because the proton conductivity in those electrolyte membranes depends on the amount of water in them. (C) 2008 Published by Elsevier Ltd, on behalf of international Association for Hydrogen Energy.