A new method for the preparation of polybenzimidazole (PBI)-based membranes, containing high concentrations of immobilized phosphonic acid groups, has been developed. The procedure used is carried out in two steps: (1) Synthesis of modified PBIs, containing 1,2-dihydroxypropyl groups and preparation of films there from; (2) Introduction of vinylphosphonic acid (VPA) and initiator (cerium ammonium nitrate) in the film, subsequent grafting of VPA from the active sites of the PBI backbone. Membranes with different length of the grafted polyvinylphosphonic acid chains were prepared. The molar ratio grafted VPA units per PBI repeating unit reaches 7.8. Proton conductivity was measured at 120 degrees C and relative humidity (RH) 20100%. For the membrane with highest concentration of phosphonic acid groups the proton conductivity was 35 mS cm1 at 100% RH and 8 mS cm1 at 20% RH. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013