In order to assess the choice of the sulphur-iodine thermochemical cycle for massive hydrogen production, a precise knowledge of the concentrations of the gaseous species (HI, I-2, and H2O) in thermodynamic equilibrium with the liquid phase of the HI-I-2-H2O ternary mixture is required, in a wide range of concentrations and for temperatures and pressures up to 300 degrees C and 50 bar. In the companion paper (Part 1) the experimental device was described, 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-H2O mixture in the 20-140 degrees C range and up to 2 bar. This (Part 2) article describes the experimental device which enables similar measurements but now in the process domain. The results concerning concentrations in the vapour phase for the HI-I-2-H2O initial mixture (with a global composition) in the 120-270 degrees C temperature range and up to 30 bar are presented. As previously, optical online diagnostics are used, based on recordings of infrared transmission spectra for HI and H2O and on UV/visible spectrometry for I-2. The concentrations measured in the vapour phase are the first to describe the vapour composition under thermophysical conditions close to those of the distillation column. The experimental results are compared with a thermodynamic model and will help us to scale up and optimize the reactive distillation column we promote for the HI section of the sulphur-iodine cycle. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.