A specially designed electrodeionization (EDI) stack was used for deep softening from simulated aqueous solutions containing low concentrations of hardness ions. Characteristics of the current-voltage (I-V) and resistance-voltage (R-V) curves of EDI and electrodialysis (ED) were compared at first, and then removal of Ca2+ ions was carried out in a continuous experiment under a constant applied voltage. In order to further investigate the abilities of EDI to remove hardness without chemical regeneration, performances of different processes were compared under various experimental conditions of applied voltage, concentrate flow rate, and feed solution composition. It was shown that with feed Ca2+ concentration of 5 mg dm(-3), the dilute resistivity of 3.45 M Omega cm was obtained when under optimized operation conditions. it was also found that applied voltage and concentrate flow rate had remarkable impacts on the migration of Call ions. A dimensionless coefficient K-t was put forward to distinguish the "enhanced transfer" and "electoregeneration" mechanisms. It was found that scaling formation due to water dissociation on the membranes surface could be avoided under reasonable operation conditions. In addition, no significant influence on final hardness rejection was revealed with multi-species of feed solutions (Ca/Mg). The experimental results have demonstrated the feasibility of deep water softening by improved EDI technology. (C) 2009 Elsevier B.V. All rights reserved.