Thermal stability, hydration and mechanical properties of thermally cross-linked sulfonated aromatic polymers (SAP) with high ionic exchange capacity (IEC) were measured and compared to untreated samples. The formation of cross-linking greatly stabilizes sulfonated polyphenylsulfone (SPPSU) in terms of thermal, mechanical, and hydrolytic degradation: it can resist in water even at a temperature of 145 degrees C with improved mechanical properties, while TGA experiments demonstrate that SPPSU membranes are stable well above 200 degrees C. Sulfonated polyethersulfone (SPES) membranes show, instead, a hydrolytic instability. (C) 2010 Elsevier B.V. All rights reserved.