The Sulphur-Iodine thermochemical cycle for hydrogen production has been investigated by ENEA in the framework of the Italian TEPSI Project whose main objective is the realization of an integrated loop plant at a laboratory scale. For the design of the separation-purification equipments, the study of vapour-liquid equilibrium characterization of the ternary HI-H2O-I-2 system is considered a key factor. The aim of the present work is to provide new experimental isobaric vapour liquid equilibrium data for this system by ebulliometry varying both temperature and solution composition. The temperature range has been extended up to about 144 degrees C, the iodine concentration range from 0.2%w/w to 90%w/w while HI weight fraction varies from 4%w/w to 67%w/w in the liquid phase. Most of the data obtained in this work are in good agreement with other experimental data retrieved from literature, which have been recorded in similar operative conditions but acquired by different procedures. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.