Mechanically strong and flexible membranes with very high local concentrations of immobilized proton-conducting phosphonic acid have been achieved by grafting poly(vinylphosphonic acid) side chains onto polysulfones. The graft copolymers were prepared by anionic polymerization of diethyl vinylphosphonate initiated from lithiated polysulfones, followed by quantitative cleavage of the ester functions. The resulting phosphonic acid units acted like monoprotic acids to indicate a high level of intramolecular interactions, and the phase-separated nature of the copolymers was shown by dual glass transitions. Fully polymeric membranes were conveniently cast from solution and showed high proton conductivities, e.g., 5 mS/cm under nominally dry conditions at 120 °C and up to 93 mS/cm under 100% relative humidity at the same temperature. The corresponding conductivities measured for Nafion 115 under the same conditions were 0.04 and 105 mS/cm, respectively. The former membranes furthermore showed high thermal stability with decomposition temperatures exceeding 300 °C under air. Additions of 2−5 wt % of a perfluorosulfonic acid polymer to the phosphonated membranes were found to reduce the water uptake significantly, thus improving the mechanical properties. The conductivity of these fully polymeric doped membranes was generally observed to be enhanced, or at least to remain at the same level, under both humidified and nominally dry conditions. The findings of this study demonstrate that phosphonated membranes with a proper macromolecular design may potentially show some important advantages in relation to the more commonly studied sulfonated membranes in fuel cell applications. [ABSTRACT FROM AUTHOR]