AbstractMembranes of phosphonated polyphenylsulfone with functionalization degree up to 88–106% were prepared by reaction in the presence of Pd(0)-complexes. Membranes of phosphonated and sulfonated polyphenylsulfone were also prepared by sulfonating previously phosphonated polyphenylsulfone. The membrane proton conductivities were evaluated by impedance spectroscopy at the temperature range between 50 and 150 °C. While sulfonated membranes had higher proton conductivity at 100% relative humidity, they swell much more than the phosphonated films in water and have a much higher permeability to water and methanol. Phosphonated films are thermally more stable.