A novel series of benzimidazole-containing sulfonated poly(arylene sulfones)s with controllable amount of basic 2,6-bis(benzimidazol-2-yl)pyridine (BIP) and sulfonic acid groups have been prepared by the copolycondensation of a new BIP-containing arylene difluoride monomer (DFSBIP) with a sulfonated arylene difluoride (DSDFS) and 4,4'-biphenol (BP). All the resulting polymers have high molecular weights, good thermal stability, and can form uniform and tough membranes by simple solution casting. Because of the strong acid-base interaction between BIP and sulfonic acid groups, ionic crosslinking networks forms that resulted in polymer membranes with good dimensional stability in water even at high temperature (e.g., 100 degrees C). The ion exchange capacity (IEC) of the polymer membranes was investigated through a new simple pH-determination method. A comparison between the experimental IEC values with the calculated ones based on the polymer structures indicated that each BIP unit interacted with one sulfonic acid group. Thus, by controlling the relative content of BIP units and sulfonate groups in the polymers, the intra- and intermolecular acid-base interactions could be readily optimized so as to achieve polymers with high IEC values, high proton conductivities as well as low swelling ratios, demonstrating good potential for proton exchange membrane applications. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1920-1929, 2009