Abstract: The following commercially available ion exchange membranes or the membranes experimentally synthesized by irradiation graft polymerization were integrated into an electrodialyzer ED and formed an electrodialysis system as shown in figure 4.1. Asahi Chemical Co.: Aciplex K102/A/102, K-172/A/172, CK-2/CA-3 Tokuyama Corp.: Neocepta CH-45T/AFS-4T, CIMS/ACS-3, CL-25T/AVS-4T Asahi Glass Co.: Selemion CMV/ASV, CMR/ASR, CMV-2/AST Japan Tobacco ＆ Salt Public Inc..: JTS-K/JTS-A The effective membrane area was maintained to 9 cm2 (otherwise 172, 500 cm2). The distance between the membranes was adjusted to 0.075 cm (otherwise 0.12 cm). Number of membrane pairs was 5 or 9. A feeding solution (seawater, desalted or concentrated seawater or 0.5 eq/dm3 NaCl) was supplied into desalting solution tank DT in figure 4.1, then it was supplied to desalting cells De at a linear velocity of 4 cm/s (otherwise 2, 5 cm/s) and at temperature of 25 0C (otherwise 29, 35, 50, 60 0C). An electric current was passed through Ag-AgCl (otherwise Ti/Pt-stainless, graphite-stainless) electrodes.