Effects of operation parameters of HI synthesis and concentration procedures on thermal efficiency in the thermochemical hydrogen production IS process were investigated based on heat/mass balance. Concentration of HI was carried out by an electro-electrodialysis (EED) cell. Transport number of proton, electroosmosis coefficient and ratio of the flow rates at the inlets of the EED cell were considered as parameters of the EED cell in different over potential and temperature difference at the heat exchangers. The parameters of the EED cell had little effect on the thermal efficiency at optimized over potential and temperature difference. The relation between these parameters and thermal efficiency was dependent on over potential or temperature difference. These parameters should be optimized by a heat/mass evaluation. Composition of the solution produced at the HI synthesis procedure influenced thermal efficiency significantly. High thermal efficiency was expected at low I-2 concentration and high HI concentration. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.