The synthesis and properties of crosslinked diblock copolymers for use as proton-conducting membranes are presented. A polystyrene-b-poly(hydroxyl ethyl methacrylate) diblock copolymer at 56 : 44 wt % was sequentially synthesized via atom transfer radical polymerization. The poly(hydroxyl ethyl methacrylate) (PHEMA) block was thermally crosslinked by sulfosuccinic acid (SA) via the esterification reaction between -OH of PHEMA and -COOH of SA. Proton nuclear magnetic resonance and Fourier transfer infrared spectra revealed the successful synthesis of the diblock copolymer and the crosslinking reaction under the thermal condition of 120 degrees C for 1 h. The ion-exchange capacity continuously increased from 0.25 to 0.98 mequiv/g with increasing SA concentration because of the increasing number of charged groups in the membrane. However, the water uptake increased up to an SA concentration of 7.6 wt %, above which it decreased monotonically (maximum water uptake similar to 27.6%). The membrane also exhibited a maximum proton conductivity of 0.045 S/cm at an SA concentration of 15.2 wt %. The maximum behavior of the water uptake and proton conductivity with respect to the,SA concentration was considered to be due to a competitive effect between the increase of ionic sites and the crosslinking reaction according to the SA concentration. All the membranes were thermally quite stable at least up to 250 degrees C, presumably because of the block-copolymer-based, crosslinked structure of the membranes. (C) 2007 Wiley Periodicals, Inc.