Phosphonic acid-bearing styrene-ethylene/butylene-styrene (SEBS) block copolymer was synthesized by bromination and subsequent palladium-catalyzed phosphonation of SEBS. The phosphonated block copolymer was characterized by spectroscopic, thermal, and conductivity measurements. The new polymer shows good ion-exchange capacity of similar to 0.7 meq/g and proton conductivity of around 2-4 mS/cm (at room temperature and 100% relative humidity) which is in good agreement with literature value of other phosphonated materials. This value was obtained despite a realtively low degree of phosphonation, demonstrating the ability of the phase separated nature of block copolymers to promote proton conductivity. (C) 2008 Wiley Periodicals, Inc.