Branched sulfonated copoly(ether ketone)s exhibit excellent properties for use as proton exchange membranes (PEMs). However, to date, the use of highly branched star-shaped sulfonated block polymers with flexible side chains as PEMs has not been reported. In this study, two novel types of star-shaped block poly(arylene ether sulfone)s containing sulfoalkyl pendant groups with 6% branching agent were synthesized for the first time. The properties of the polymer with hydrophilic segments surrounded by hydrophobic segments (SPAES-6s) and of the polymer with hydrophobic segments surrounded by hydrophilic segments (SPAES-6f) were investigated, with a focus on their potential use as PEMs. The block polymers SPAES-6f and SPAES-6s exhibited higher proton conductivities (0.42 S cm(-1) and 0.30 S cm-1 at 80 degrees C) than those of the random polymer SPAES-6r and Nafion 117, and SPAES-6s exhibited better oxidative stability (435 min) and dimensional stability than SPAES-6f. The results indicate that the highly branched sulfonated block poly(arylene ether sulfone)s with sulfoalkyl pendant groups are good candidate PEM materials for fuel cell applications. (C) 2015 Elsevier B.V. All rights reserved.