End-group crosslinked sulfonated poly(arylene sulfide nitrile) (XESPSN) membranes are prepared to investigate the effect of crosslinking on the properties of sulfonated aromatic polymer membranes at elevated temperatures (>100 degrees C). The morphological transformation during annealing and crosslinking is confirmed by atomic force microscopy. The XESPSN membranes show outstanding thermal and mechanical properties compared to pristine and non-crosslinked ESPSN and Nafion (R) up to 200 degrees C. In addition, the XESPSN membranes exhibit higher proton conductivities (0.011-0.023 S cm(-1)) than the as-prepared pristine ESPSN (0.004 S cm(-1)), particularly at elevated temperature (120 degrees C) and low relative humidity (35%) conditions due to its well-ordered hydrophilic morphology after cross-linking. Therefore, the XESPSN membranes demonstrate significantly improved maximum power densities (415-485 mW cm(-2)) compared to the ESPSN (281 mW cm(-2)) and Nafion (R) (314 mW cm (2)) membranes in single cell performance tests conducted at 120 degrees C and 35% relative humidity. Furthermore, the XESPSN membrane exhibits a much longer duration than the ESPSN membrane during fuel cell operation under a constant current load as a result of its improved mechanical and thermal stabilities. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.