In the framework of the massive production of hydrogen using the sulfur-iodine thermochemical cycle, the design of the reactive distillation column, chosen by CEA for the HIx section, requires the knowledge of the partial pressures of the gaseous species (HI, I(2), H(2)O) in thermodynamic equilibrium with the liquid phase of the HI-I(2)-H(2)O ternary mixture in a wide range of concentrations up to 270 degrees C and 50 bar. In the first of these two companion papers, we describe the experimental device which enables the measurement of the total pressure and concentrations of the vapour phase (and thus the knowledge of the partial pressures of the different gaseous species) for the HI-I(2)-H(2)O mixture in the 20-140 degrees C range and up to 2 bar. This device is used to carry out a large set of experiments investigating various mixtures with optical on-line diagnostics (FTIR for HI and H(2)O, UV-visible for I(2)). This leads to the determination of the concentrations in the vapour phase for many experimental conditions, results of which are given in this paper. The companion paper (part 2) describes the experimental device which enables measurements of the total pressure and species concentrations in the vapour phase in the process domain. The experimental results obtained using both experimental devices, a premiere as far as we know, will be used in the future to establish a new thermodynamic model which will allow the reliability of the evaluation of the iodine part of the iodine/sulfur cycle to be increased. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.