Sulfonated polysulfone (SPSU) membranes with different sulfonation levels have been prepared and evaluated as proton exchange membranes in polymer electrolyte fuel cells (PEFC). The membranes have been characterized by ion-exchange capacity (IEC), thermal analysis, proton conductivity and single cell performance. The introduction of sulfonic groups in the base polymer produces an increase in glass transition temperature (T-g) from 190 to about 200-220 degreesC and a thermal decomposition of polymer at lower temperature. Proton conductivity of 4.3 x 10(-2) S . cm(-1) at 80 degreesC on a SPSU membrane with IEC = 1.25 meq/g was obtained. Membrane/electrodes assemblies (MEAs) prepared with SPSU gave, in single cell tests at 80 degreesC, power densities of 400 and 500 mW . cm(-2) in H-2/air and H-2/O-2, respectively.