The electro-clectrodialysis (EED) experiment was carried out to concentrate HI from HIx (HI-H2O-I-2 mixture) solution, which may be achieved by thermochemical water splitting IS (iodine-sulfur) process. The molar ratios of the HIx solution were HI:H2O:I-2 = 1:6.0:0.5 (I-2-0.5 solution), 1:5.8:1 (I-2-1 solution) and 1:6.4:2 (I-2-2 solution). EED cell are composed of the collector, electrode and electrolyte. The carbon plate, Nafion 117, which was cation exchange membrane and the activated carbon cloth used as an collector, electrolyte and electrode, respectively. In the catholyte, the HI mole fractions increased and 12 mole fractions decreased by EED operation. The cell voltage reached to 0.24 V, 0.29 V and 0.40 V for I-2-0.5 solution, I-2-1 solution and I-2-2 solution after 10 h EED operation, respectively. The electro-osmosis coefficient decreased from 1.49 mol/Faraday to 0.93 mol/Faraday and proton transport number increased from 0.74 to 0.84 with an increase of iodine molar ratio. It has been observed that the rate of enhancement in HI concentration is also affected by membrane properties. (c) 2007 Elsevier B.V. All rights reserved.