Novel sulfonated polyimide copolymers as electrolytes for high-temperature fuel cell applications are reported. A series of sulfonated polyimide copolymers (SPIH-X; X refers to molar percentage of fluorenyl content) containing 0-60 mol % of fluorenyl groups as hydrophobic component were synthesized, of which electrolyte properties were investigated and compared to those of the perfluorinated ionomer (Nafion 112). High-molecular-weight copolymers with good film-forming capability were obtained. Thermal stability with decomposition temperature of ca. 280 degreesC and no glass transition temperature was confirmed for the copolymers. SPIH shows unique water uptake behavior with the maximum value of 57 wt % at X = 30. Water molecules absorbed in the electrolyte membrane with this specific composition do not evaporate easily so that the high proton conductivity of 1.67 S cm(-1) was obtained at 120 degreesC and 100% RH. The branching and cross-linking of SPIH-30 were carried out by applying 2 mol % of trifunctional monomer (melamine) in the polymerization and by electron beam irradiation upon the membrane. The branching and cross-linking are effective to improve oxidative stability and mechanical strength. Although the proton conductivity decreases slightly by the branching and cross-linking, it still remains at the comparable level to that of Nafion 112.