Sulfonation of polysulfone strongly affects both protonic conductivity and lifetime of composite polyelectrolytes. Viscosimetric comparisons showed the effect of the sulfonating agent. Indeed, chlorosulfonic acid leads to chain cleavage while its trimethyl silyl ester does not affect the polymer backbone. Sampling aliquots allowed the sulfonation yield to be followed by H-1 NMR. From this study, one might infer that the conversion should not exceed 70% of the theoretical yield. Viscosimetric measurements performed on the same aliquots demonstrated that, even after 72 h reaction time, chain cleavage did not occur. Filling of sulfonated polysulfone with 8% of phosphatoantimonic acid resulted in a conductivity trebling 0.06 versus 0.02 S cm(-1) at 80 degreesC and 98% of relative humidity (RH). Electrochemical performances, thermo-mechanical stability and low cost make this composite membrane an attractive material for proton exchange membrane fuel cell (PEMFC).