Among many thermochemical cycle hydrogen production methods, the iodine sulfur (IS) cycle, originally proposed by General Atomics is expected to be a clean, economical, and sustainable method of large-scale hydrogen production using nuclear heat at high temperature. However, there still exist some technical problems to be solved before realization of industrial hydrogen production using the IS process. One such problem is purification of Bunsen reaction products. The Bunsen reaction products in the IS cycle, H2SO4 and HIx phases, containing minor acids of HI and H2SO4, respectively, should be purified in order to avoid occurrence of side reactions. In this paper, simulation study on the purification of H2SO4 and HIx phases were carried out at pressure both higher and lower than 1 atm by a chemical process simulator ESP with the thermodynamic database based on the Mixed Solvent Electrolyte model. The results showed that purification of H2SO4 under subatmospheric pressure could increase the conversion of HI, improve the selectivity of SO2, and promote I-2 and SO2 conversion products distributing in the vapor phase. Although purification of HIx phase under subatmospheric pressure could also increase the conversion of H2SO4, it did not facilitate the occurrence of the reverse Bunsen reaction. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.