A kinetic investigation of the water-uptake of Nation 117 membranes after different hydrothermal and thermal treatments was performed. Long equilibration times (150-225 h) and a decrease of kinetic rate with the increasing of temperature were found. This behaviour suggests that the kinetics of the water-uptake is the result of two distinct processes: one very fast that can be attributed to the time of osmotic equilibration necessary for the water diffusion within the thin membrane and the other, very slow, that has been associated with a slow modification of the Nation conformation with the temperature. The irreversibility of the hydration process with the temperature was related to the irreversibility of the conformational changes. The memory of the thermal treatment is due to the fact that ionomers are essentially constituted as an amorphous matrix in which some microcrystalline phases are embedded. All the amorphous ionomers can give metastable phases that can appear kinetically stable although, in fact, they are thermodynamically unstable. Previous osmotic models for ionomers were re-visited in order to relate the water-uptake isotherms of the various treated samples to the counter-elastic force of their matrix. It was found that an index proportional to the counter-elastic force of the matrix can be simply derived by determining the water-uptake of the samples in liquid water at 20 degrees C. Furthermore, some useful relations between this index and tensile modulus and the shape of water-vapour sorption isotherm of the samples are reported and discussed. We believe that the researches on the modifications of PFSA membranes by thermal and hydrothermal treatments are very important: (1) in order to have a better understanding of the fundamental properties of this important class of membranes; (2) for obtaining reliable comparisons between the properties of different PFSA membranes and (3) for improving the stabilization of the Nafion membranes at temperatures higher than 90 degrees C. (C) 2007 Elsevier B.V. All rights reserved.