The terminally-crosslinked sulfonated poly(ether sulfone) block copolymer (SPES-b) was developed for use as a proton exchange membrane in DMFC applications. The properties of the membrane prepared from SPES-b were compared with those of its non-crosslinked counterpart (SPES-bn) and Nafion (R)-117. The membrane displayed a well-defined morphology due to phase separation between the hydrophobic and hydrophilic ionic units in the SPES-b block copolymer. The terminally-crosslinked SPES-b membrane yielded a high proton conductivity of 0.09 S/cm and a low methanol permeability of 5.3 x 10(-8) cm(2)/s at 20 degrees C, and the membrane displayed excellent dimensional, mechanical, hydrolytic, and oxidative stabilities. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.