We developed an improved thermo-physical model for the hydrogen-iodide (HI) VLE and decomposition behavior in the iodine-sulfur (IS) cycle. We reproduced the Neumann's modified NRTL model by an optimization scheme from the experimental data that he used. Then, we improved the model by correcting his unphysical assumption for the non-randomness parameter, and used the two-step equilibrium approach for the Hi decomposition modeling. The KAIST model (improved Neumann's model) with a new set of binary parameters predicts the total pressure of HI solution within 2.98% of RMS error for the HI-H2O binary mixture, and within 7% of RMS error for the HI-H2O-I-2 ternary mixture. We also proposed a simple logic to predict the liquid-liquid phase separation condition within 2.07% of relative deviation in the temperature range of 70-149 degrees C. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.