PEMs prepared from sulfonated poly(ether ether ketone) and cross-linked with simple polyols (ethylene glycol and glycerol), exhibited enhanced strength, stability in hot water and high proton conductivity. The properties and chemical structure of the cross-linked membranes were studied using TGA, H-1 NMR and FTIR. These data suggest that ethylene glycol and glycerol molecules do not link neighbouring main chains via sulfonic acid functions, but rather form alcohol-ether oligomers, bonded to -SO3- group and generating an interpenetrating network which interacts with the main chains. The thermal stability of cross-linked membranes is inferior to that of non-cross-linked ones. It is shown that splitting off of the sulfonic acid groups occurring above 200 degrees C is accompanied by main chain degradation in both initial and cross-linked membranes. The fact that the degradation temperature exceeds 200 degrees C suggests that this could-be a promising and effective approach to cross-linking sulfonated membranes for FC application. (c) 2006 Elsevier B.V. All rights reserved.