Sulfonated multiblock copoly(ether sulfone)s applicable to proton exchange membrane fuel cells (PEMFCs) were synthesized by the coupling reaction of the hydroxyl-terminated hydrophilic and hydrophobic oligomers with different lengths in the presence of highly reactive decafluorobiphenyl (DFB) as a chain extender to investigate the influence of each length on the membranes' properties, such as water uptake, proton conductivity, and morphology. Multiblock copolymers with high molecular weights (M-n > 50,000, M-w > 150,000) were obtained under mild reaction conditions. The resulting membranes demonstrated good oxidative stability for hot Fenton's reagent and maintained high water uptake (7.3-18.7 wt %) under a low relative humidity (50% RH). Proton conductivity of all membranes at 80 degrees C and 95% RH was higher than that of Nafion 117 membrane, and good proton conductivity of 7.0 x 10(-3) S/cm was obtained at 80 degrees C and 50% RH by optimizing the oligomer lengths. The surface morphology of the membranes was investigated by tapping mode atomic force microscopy (AFM), which showed that the multiblock copolymer membranes had a clearer surface hydrophilie/hydrophobic-separated structure than that of the random copolymer, and contributed to good and effective proton conduction. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7332-7341, 2008