The reaction equilibrium and phase equilibrium in H2SO4 and HIx phases produced by the Bunsen reaction of the iodine-sulfur thermochemical hydrogen production process were examined using a chemical process simulator, ESP, with a thermodynamic database based on the mixed solvent electrolyte model. At temperatures lower than ca. 110 degrees C, the reaction of HI and H2SO4 produced elemental sulfur in both phases. At higher temperatures, the reverse Bunsen reaction occurred, and SO2 was produced in the H2SO4 phase. In the HIx phase, conversely, SO2 formation predominated in a narrow temperature range and H2S was produced with the increase in temperature. The presence of N-2 gas lowered the temperature of the predominant reaction change. A feed of O-2 for purification was proposed to suppress the consumption of objective components in the H2SO4 phase purification, and an O-2 feed to the HIx phase for the suppression of H2S and S impurities was proposed by the simulation.