In recent years, the Hybrid Sulphur (HyS) and Sulphur Iodine (SI) cycles have emerged as promising routes for the massive scale thermochemical production of Hydrogen from water. Common to these two cycles is a high temperature stage involving the decomposition of sulphuric acid to sulphur dioxide, water and oxygen. The work considered here focuses on the oxygen separation stage of the process where, in order for a science based design approach to be taken, thermodynamic data are required for multicomponent phase equilibrium relationships between the decomposition products; H(2)O-SO(2)-O(2). A method of making flash calculations, useful in the separation equipment design, has been proposed and coded into Mathematica (R). Further, a vapour liquid equilibrium still has been designed and built to make measurements of the multicomponent solubility of sulphur dioxide and oxygen in water. The experiments presented here concentrate on the binary SO(2)-H(2)O system, covering a wider range of pressures than that found in the literature. Solubility, expressed in molality, was obtained at 25 and 40 degrees C and pressures ranging from 0.2 to 3.6 atm. Good agreement is seen between the experimental data and the model at lower pressures, however improvements are needed at higher pressure. Preliminary data for the ternary system at 40 degrees C is presented, showing good agreement with the multicomponent model. Future work will focus on the three component system. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.