Proton conductivity and hydration of sulfonated polyethersulfone (SPES) membranes, with ion exchange capacity of 1.31 meq g(-1), is determined under different conditions of temperature, in the range 70-120 degrees C, and relative humidity (RH) in the range 50-90%. Conductivity measurements are performed by the impedance technique, and the impedance data are analyzed on the basis of a simple equivalent circuit in order to compare the evolution of the membrane conductivity with that of the electrical capacitance of the electrode-electrolyte interface during heating and cooling runs. Temperature cycling at constant RH, as well as RH cycling at constant temperature,give rise to hydration hysteresis and to the concomitant conductivity hysteresis. The highest conductivity (4.5.10(-2) S cm(-1)) is measured at 100 degrees C-90% RH with 8.4 water molecules per sulfonic group. The hysteresis associated with temperature cycling is avoided by filling SPES with zirconium phosphate which makes hydration easier at low temperature. (C) 2012 Elsevier B.V. All rights reserved.