A series of poly(arylene ether sulfone)s containing pendant sulfonic acid groups have been prepared by an aromatic substitution polymerization reaction using 4,4-difluorodiphenylsulfone, 6,7-dihydroxy-2-naphthalenesulfonate, and various hydroxyl terminated monomers in the presence of potassium carbonate. The synthesized sulfonated polymers have been characterized by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, gel permeation chromatography, ion exchange capacity, thermogravimetric analysis, and proton conductivity measurements. With a molecular weight of 50,000 g/mol and an ion exchange capacity of similar to 1.17 meq./g, these polymers are thermally stable up to 250 degrees C. They are found to exhibit better performance at 65 and 80 degrees C in direct methanol fuel cells than Nafion 115 membrane despite lower proton conductivity due to a significantly lower methanol crossover. (C) 2008 Elsevier B.V. All rights reserved.