A series of crosslinked polymer electrolyte membranes with controlled structures were prepared based on poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymer and a sulfonated monomer, 2-sulfoethyl methacrylate (SEMA). SBS membranes were thermally crosslinked with SEMA in the presence of a thermal-initiator, 4,40-azobis(4-cyanovaleric acid) (ACVA), as confirmed by FT-IR spectroscopy. The water uptake and ion exchange capacity (IEC) of membranes increased almost linearly with SEMA concentrations due to the increase of SO(3)(-) groups. However, the proton conductivity of membranes increased linearly up to 33 wt % of SEMA, above which it abruptly jumped to 0.04 S/cm presumably due to the formation of well-developed proton channels. Microphase-separated morphology and amorphous structures of crosslinked SBS/SEMA membranes were observed using wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). The membranes exhibited good mechanical properties and high thermal stability up to 250 degrees C, as determined by a universal testing machine (UTM) and thermal gravimetric analysis (TGA), respectively. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 3283-3291, 2011