Highly sulfonated poly(ether sulfone)s with densely populated flexible acid side chains were prepared for fuel cell applications by polycondensation of 3,3'-dihydroxybenzidine with bis(4-fluorophenyl)sulfone and 4,4'-biphenol, followed by postsulfonation using 1,4-butanesultone at room temperature. The sulfonated polymers gave tough, flexible, and transparent membranes by solvent casting. The membranes had high ion exchange capacity (IEC) values (2.47-2.95 mequiv/g) and displayed good proton conductivities in the range of 13.90-20.90 x 10(-2) and 1.08-2.21 x 10(-2) S/cm at 95% and 35% relative humidity (RH) (80 degrees C), respectively. In particular, the S-PES-55 membrane with the highest IEC value showed higher or comparable proton conductivity than that of Nafion 212 in the range of 35-95% RH. The morphologies of these membranes were investigated by TEM analysis, which exhibited well-connected hydrophilic channels due to their high IEC values and densely populated flexible acid side chains. In contrast with many reported highly sulfonated polymers, the membranes showed good dimensional stability regardless of their high IEC values. (C) 2012 Crown Copyright and Elsevier B.V. All rights reserved.