The electro-electrodialysis (EED) experiments for thermochemical water-splitting IS (iodine-sulfur) process was carried out to concentrate HI from HIx (HI-H2O-I-2 mixture) solution. The molar ratios of the HIx solution were HI: H2O:I-2 = 1: 6: 2 (I-2-2 solution) and 1:6:3 (I-2-3 solution). EED cell is composed of the collector, electrode and electrolyte. The graphite plate, Nation 117 which was cation exchange membrane and the activated carbon cloth used as an collector, electrolyte and electrode, respectively. Two types of electrode which had the different surface area (type B electrode: 1400, type C electrode: 700 m(2)/g) were selected to compare the ratio of the HI concentration. The electrical energy demand only for electrode reaction through the EED operation using type B electrode was 300 J/h lower than that using the type C electrode in I-2-2 solution. In all EED experimental conditions, HI concentration increased with increasing operation time that meant the concentrated HI from HIx solution. Moreover, it could be confirmed that the quasi-azeotropic composition have been overcome during the EED operation. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.