First results on the measurement of total and partial pressures over the ternary system HI-I-2-H2O are reported. Using original optical online measurements, data on the gas phase speciation are obtained which will help to scale and optimize the reactive distillation column we promote for the HI section of the sulphur-iodine cycle. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.