Proton-conductive membranes, exhibiting high chemical and thermomechanical resistance, have been obtained by s-PS films with the beta crystalline phase and a sulfonated amorphous phase. These membranes can be obtained by a solid-state procedure on delta form films, which allows an easy and uniform sulfonation of the phenyl rings of the amorphous phase and preserves the crystalline phase, followed by suitable thermal treatments leading to the delta -> beta crystal-to-crystal transition. The high degree of sulfonation of the amorphous phase makes this phase highly hydrophilic and proton conductive while the presence of the high melting and thermodynamically stable beta phase assures high chemical and then momechanical stability. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.