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 degrees C for 1.5 h 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 40 mol% sulfonated moiety, by cross-linking the tensile strength increased from 39 MPa (dry) or 21 MPa (wet) to 44 MPa (dry) or 30 MPa (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 3 8 wt% and the methanol permeability from 7.0 x 10(-7) to 1.6 x 10(-7) cm(2) s(-1) (30 degrees C). The proton conductivity, however, did not decrease too much (from 0.076 to 0.043 S cm(-1) in water at 30 degrees C. (C) 2007 Elsevier B.V. All rights reserved.