A model for the Bunsen section of the Sulfur-Iodine thermo-chemical cycle is proposed, where sulfur dioxide reacts with excess water and iodine to produce two demixing liquid aqueous phases (H(2)SO(4) rich and HI rich) in equilibrium. Considering the mild temperature and pressure conditions, the UNIQUAC activity coefficient model combined with Engels' solvation model is used. The complete model is discussed, with HI solvation by water and by iodine as well as H(2)SO(4) solvation by water, leading to a very high complexity with almost hundred parameters to be estimated from experimental data. Taking into account the water excess, a successful reduced model with only 15 parameters is proposed after defining new apparent species. Acids total dissociation and total H(+) solvation by water are the main assumptions. Results show a good agreement with published experimental data between 25 degrees C and 120 degrees C. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.