A new and facile approach has been developed for the preparation of cross-linked sulfonated poly(sulfide sulfone) (SPSSF) membranes. The cross-linking reaction was performed by immersing the SPSSF membranes into polyphosphoric acid at 180°C for 1.5h and the cross-linking bond was the very stable sulfonyl group. Cross-linking significantly improved the membrane performance, i.e., the cross-linked membranes showed better mechanical properties, lower water uptake and lower methanol permeability than the corresponding uncross-linked ones, while reasonably high proton conductivity was maintained. For example, for the membrane containing 40mol% sulfonated moiety, by cross-linking the tensile strength increased from 39MPa (dry) or 21MPa (wet) to 44MPa (dry) or 30MPa (wet) and the elongation at break from 17% (dry) or 18% (wet) to 65% (dry) or 21% (wet), while the water uptake was reduced from 74 to 38wt% and the methanol permeability from 7.0×10 −7 to 1.6×10 −7 cm 2 s −1 (30°C). The proton conductivity, however, did not decrease too much (from 0.076 to 0.043Scm −1 in water at 30°C).