This paper presents a laboratory experiment for the study of the liquid-vapor equilibrium Of HI-I(2)-H(2)O ternary mixtures that are involved in the iodine sulphur thermochernical cycle for the production of hydrogen. The apparatus is based on a heated tantalum pressurized vessel equipped with temperature and pressure transducers and carrying windows for optical soundings, It enables recordings of the infrared transmission of the vapor with a Fourier Transform Spectrometer (FTS). The spectra show the signatures of HI and H(2)O through rovibrational absorption transitions in both the 4200-5800 cm(-1) (1.7-2.4 mu m) and 6200-7800 cm(-1) (1.3-1.6 mu m) regions. In order to analyze the FTS recordings, a spectra calculation software and a dedicated database of spectroscopic parameters have been built. Using these tools, the HI and H(2)O amounts in the vapor are determined from least square fits of the experimental spectra in both regions. it is first shown that the measured and the calculated transmissions are in good agreement. Furthermore, the species amounts determined independently in the two spectral regions are very consistent, confirming the quality of the optical soundings. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.