Sulfonated poly(arylene ether sulfone) copolymers containing carboxyl groups are prepared by an aromatic substitution polymerization reaction using phenolphthalin, 3,3'-disulfonated-4,4'-dichlorodiphenyl sulfone, 4,4'-dichlorodiphenyl sulfone and 4,4'-bisphenol A as polymer electrolyte membranes for the development of high temperature polymer electrolyte membrane fuel cells. Thin, ductile films are fabricated by the solution casting method, which resulted in membranes with a thickness of approximately 50 mu m. Hydroquinone is used to crosslink the prepared copolymer in the presence of the catalyst, sodium hypophosphite. The synthesized copolymers and membranes are characterized by (1)H NMR, FT-IR, TGA, ion exchange capacity, water uptake and proton conductivity measurements. The water uptake and proton conductivity of the membranes are decreased with increasing the degree of crosslinking which is determined by phenolphthalin content in the copolymer (0-15 mol%). The prepared membranes are tested in a 9 cm(2) commercial single cell at 80 degrees C and 120 degrees C in humidified H(2)/air under different relative humidity conditions. The uncrosslinked membrane is found to perform better than the crosslinked membranes at 80 degrees C; however, the crosslinked membranes perform better at 120 degrees C. The crosslinked membrane containing 10 mol% of phenolphthalin (CPS-PP10) shows the best performance of 600 mA cm(-2) at 0.6 v and better performance than the commercial Nafion (R) 112 (540 mA cm(-2) at 0.6 V) at 120 degrees C and 30% RH. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.