The iodine-sulfur (IS) thermochemical water splitting cycle is one of the most promising massive hydrogen production approaches using nuclear or solar energy. Bunsen reaction is a crucial reaction of the process by which sulfuric and hydriodic acids form. The simultaneously phase separation of the Bunsen production products, composed of HI, I-2, H2SO4, and H2O are essential for the smooth operation of the process. In this work, the phase separation characteristics of the quaternary mixtures were studied at elevated temperatures of 40-80 degrees C to clarify the phase separation characteristics. The critical conditions of the phase separation and iodine solubility in the mixture, as well as the effects of the composition and temperature on the purity of the two phases, were investigated. Favorable concentration range of each component in the quaternary solutions at various temperatures was obtained and presented in a tetrahedron diagram. The solution separates into two liquid phases spontaneously without iodine precipitation within the range. The amount of I-2 should be as high as possible, with no I-2 precipitation, to keep impurities at minimum content in the two phases. The results of this study offer technical basis for the design and operation of the IS process. (c) 2012 Elsevier B.V. All rights reserved.