The phase separation of the Bunsen products mixture containing HI, I-2, H2SO4, and H2O is an essential subsection of the iodine-sulfur hydrogen production process (I-S process). It is also the pre-condition of the continuous operation of the I-S process. This experimental study was conducted at 20 degrees C using jacketed glass vessels to clarify the characteristics of the phase separation of the quaternary mixture. The critical conditions of the phase separation, iodine solubility in the mixture, and intermiscibility of the constituents in the solution were investigated. The favorable concentration range of each component in the Bunsen products mixture was illustrated using a tetrahedron diagram. Within the concentration range, the solution separates spontaneously into two liquid phases without iodine precipitation. In addition, a computer program was developed to easily ascertain the phase state of the quaternary mixture with a given composition. (C) 2012 Elsevier B.V. All rights reserved.