Different ionomers were obtained by sulfonation of a commercial polystyrene (PS), using acetyl sulfate as reagent. The sulfonation was assessed by FTIR and XPS spectroscopies and the thermal and mechanical properties were deduced from DMA measurements. The acid form of the ionomers was characterized by means of H-1 and C-13 MAS-NMR spectroscopies. Polymer hydration under controlled humidity atmosphere was analyzed by gravimetric, NMR and complex impedance techniques. In the hydrated state, two stages associated with formation of hydromium (H3O+) species and proton hopping between adsorbed water molecules were deduced from NMR data. Both processes are responsible for important changes detected in mechanical properties and conductivity of sulfonated polymers during hydration processes. (c) 2006 Elsevier B.V. All rights reserved.