A multistage electrodialyzer having an enriching section and a stripping one has been developed to separate monovalent cation species in the aqueous solution. A feed solution containing LiCl and KCl is supplied to the middle stage of the dialyzer, and a solution of HCl is supplied to the cathode stage. Stages from cathode to feed are the enriching section and those from feed to anode are stripping section. Aqueous solution flows from the cathode stage toward the anode stage as a reflux flow. The ratio K+/Li+ in the enriching product increased with the increase in reflux flow rate and decreased with the increase in current density, while the ratio Li+/K+ in the stripping product increased with the current density and decreased with the reflux flow rate. Simulation by a stage to stage calculation was carried out by use of the operating equation based on mass balance and the relation of permselectivities alternatively. The calculated results were in good agreement with the experimental ones, and indicated that a large yield of product as well as a large concentration ratio can be achieved by increasing stage number. (C) 2004 Elsevier Ltd. All rights reserved.