In the framework of the optimization of the sulfur-iodine thermochemical cycle for massive hydrogen production, investigations were performed in order to characterize the liquid phase (HIx and H(2)SO(4(aq)) phases) separation of solutions resulting from Bunsen reaction. Quaternary H(2)SO(4)/HI/I(2)/H(2)O mixtures were prepared at 308 K with different relative proportions of reactants and the chemical composition of each of the two phases formed was analyzed. An increase in iodine concentration and a decrease in water concentration appeared to improve the liquid-liquid equilibrium phase separation. However, a too low concentration of water also promoted the formation of byproducts. An increase in the [H(2)SO(4)]/[HI] ratio tended to favor the separation and seemed to lead to a dehydration of the HIx phase. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.