The effect of sulfuric acid on the concentration of HIx solution by electro-electrodialysis (EED) was examined for the thermochemical water-splitting iodine-sulfur process. Presence of sulfuric acid in the anolyte HIx solution did not affect the concentration behavior. However, sulfuric acid in the catholyte solution caused side re action(s) producing whitish precipitates, which indicates that the sulfur compound should be removed prior to the EED operation. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.