Proton exchange membranes with pendent phosphonic acid groups were synthesized by pre-irradiation grafting from vinylbenzyl chloride onto FEP and ETFE films with subsequent Arbuzov phosphonation, Free phosphonic acid groups, which are necessary for proton conductivity, were obtained by acid ester hydrolysis. The phosphonated membranes were characterized by phosphonation degree, FTIR-spectroscopy, ion exchange capacity (IEC), oxidation stability, swelling properties, and thermal properties (TGA).