The phosphoric acid doping levels of homogeneous membranes from blends of polybenzimidazole (PBI) with sulfonated polysulfones (SPSF) were established and found to be dependent on the PBI/SPSF ratio, on the presence of 'non-bonded' PBI, on the sulfonation degree of the SPSF as well as on the impregnation temperature. Optimum parameters for using these blends in polymer electrolyte fuel cells were set. Ionic conductivity of these acid doped membranes at 160 degreesC was Found considerably higher than the conductivity of acid doped PBI membranes.