The iodine-sulfur (IS) thermochemical process is one of the most prospective, efficient, CO2-free, massive hydrogen production approaches that use nuclear or solar energies. Among the three reactions composed of the IS cycle, the Bunsen reaction is crucial to the smooth operation of the continuous closed cycle. In the Bunsen reaction, sulfuric and hydriodic acids are produced by the reaction of recycled products of the decomposition of these two acids, In this work, a Bunsen reaction under simulated closed-cycle conditions, i.e., reaction between I-2/HI/H2O solution and SO2, was investigated. The effects of reaction conditions such as SO2 flow rate, HI acid concentration, I-2/HI molar ratio, and temperature on the characteristics of the Bunsen products were examined. These characteristics were phase separation, phase volume ratio, and compositions of HIx and H2SO4 phases. The concentration of HI acid and the I-2/HI molar ratio of the initial solution, but not the flow rate of SO2, were found to affect the phase states and compositions of Bunsen reaction products. The phase states and compositions of the products were predicted or calculated with self-built empirical models. Results well agreed with the experimental results for phase states and HI composition. All these findings validated the reliability of the models and offered crucial reference and guidance for the operation of the closed-cycle IS process. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.