We prepared proton exchange membranes of sulfonated poly(phenylsulfone) [SPPSU] cross-linked with ethylene glycol (EG) or glycerol (GL). The cross-linking reaction was performed in an SPPSU aqueous solution including 0-12 molecules/repeating unit; R.U. of the cross-linker at 180-200 degrees C to afford water-insoluble SPPSU membranes even in boiling H2O. The characteristic properties of the SPPSU membranes, such as the number of cation exchange sites, water retention abilities, mechanical properties, and proton conductivities, can be easily optimized by selecting the amount and type of cross-linkers and the reaction temperature. The SPPSU membranes had tensile strengths of up to 43.5 MPa, moduli of up to 1.9 GPa, and elongations at break of up to 30.2%. An SPPSU membrane prepared at 180 degrees C using EG as a cross-linker exhibited a maximum proton conductivity of 0.23 S/cm at 120 degrees C and 90% relative humidity (RH), and the value remained at 0.11 S/cm even at 40 degrees C. A maximum residual weight of 98% was achieved for SPPSU membranes prepared at 200 degrees C using GL as the cross linker under oxidation conditions at 80 degrees C for 1 h, and they exhibited good chemical stability.