Poly(arylene ether sulfone) are well known engineering thermoplastics with many desirable properties such as excellent mechanical strength, high thermal and thermo-oxidative stabilities. Partially fluorinated poly(arylene ether sulfone) copolymers containing cross-linkable moieties were prepared via nucleophilic substitution reaction. The chemical structures of the synthesized copolymers were confirmed by 1H NMR and FTIR-ATR spectra. Cross-linked polymers were obtained by thermal curing of the cross-linkable copolymers. The thermal properties of the cross-linked polymers analyzed by TGA and DSC were considerably improved due to the formation of three-dimensional networks between polymer chains. The cross-linking degree of the cross-linked copolymers was determined by a solvent extraction. The formation of cross-linking structures in the polymer matrix can be important for proton exchange membrane fuel cells with improved properties.