Poly(2-dimethylamino ethyl methacrylate) (PDMAEMA) is a weak cationic polyelectrolyte in aqueous solution which was incorporated in the porous poly(acrylonitrile and 2-dimethylamino ethyl methacrylate) (PAN-DMAEMA) membrane. The signals of the formation of ion-pair complexes between protonated DMAEMA groups and anions of the added salts were clearly observed in the X-ray photoelectron spectroscopy (XPS) spectra of the PAN-DMAEMA membrane. Since the electrostatic interactions between propotated DMAEMA groups and anions are highly dependent on the valency of anions, which influences the conformation of PDMAEMA and pore sizes of membrane, ion-specific flux was found for the PAN-DMAEMA membrane. At the same salt concentration, water fluxes of the PAN-DMAEMA membrane are enhanced in the order of trivalent > divalent > monovalent anions.