In the development of the Hybrid Sulfur process (HyS) for generation of hydrogen using a pebble bed modular reactor (PBMR) as the high temperature heat source, optimal process economics depend in part on efficient heat recovery within the sulfuric acid decomposition system. Simulation of this system in turn depends on reliable thermodynamic modeling of the complex, high-temperature and -pressure processes. Current simulation efforts using the CHEMCAD™ chemical process simulator have established a basis of vapor-liquid equilibrium thermodynamic properties for the sulfuric acid-sulfur dioxide-water system that reliably reproduces literature data for the unit operations involved.