Grafting of (4-bromomethyl)benzenesulfonate onto polybenzimidazole (PBI) leads to a proton exchange polymer of variable degree of sulfonation. At high degrees of sulfonation, the membranes display a phenomenon of dehydration and contraction, which is shown to be due to the presence of a chain cross-linking strong hydrogen bond network. Immersion of proton form benzylsulfonate-grafted membranes in aqueous organic or inorganic base is sufficient to disrupt this network, and lead to rehydration and return to a swollen and flexible state, and base-doped N-benzylsulfonate-grafted PBI (PBI-BzSH) have a conductivity of 2 x 10(-2) S cm(-1) at 25 degreesC and 100% relative humidity. Exchange of sodium form N-benzylsulfonate-grafted PBI with H+ takes place with three points of neutralisation, and in the pH range 5.5-12.4, this polymer exists predominantly in a zwitterionic form.