A series of new polyimides bearing simultaneously sulfonic acid groups and benzimidazole rings in different proportions have been synthesised by one pot high temperature polycondensation. The composition of the copolymers, as well as the distribution of the ionic sites along the polymer chain, has been changed systematically in order to study their relation with polymer properties. The incorporation of sulfonic side groups in the structure improves the solubility in polar organic solvents. In turn, the presence of sulfonic acid moieties impairs thermal resistance. Membranes based on the synthesised polyimides were prepared by solution casting and their water uptake as well as proton conductivity were determined. (C) 2008 Elsevier Ltd. All rights reserved.