A new type of ion exchange membrane was investigated for the purpose of applying electrodialysis process. The cation exchange membrane was prepared by sulfonation of the home-made block copolymers of polysulfone (PSf) and polyphenylenesulfidesulfone (PPSS). The lowest measured membrane area resistance, equilibrated in 2 M (mol/dm(3)) KCI aqueous solution, was 2.5 Omega cm(2). The cation exchange capacity reached up to 1.9 meq/(g-dry-resin), and the transport number of K+ was in the 0.77-0.87 range.