Material, energy, and entropy balances, which depend only on stream conditions and flows entering and leaving a system, have been used to evaluate different scenarios for thermochemical decomposition of water to manufacture hydrogen using the Sulfur-Iodine cycle. Energy efficiencies have been found for idealized systems with variable stream amounts, as well as for a common flowsheet, to locate the greatest effects on energy requirements and inefficiencies. Aspen Plus(R), OLI Engine, and ProSimPlus property models have been used on the sections of a General Atomics process to reveal the effects of differences in computed energies and entropy generation. While the calculated efficiencies are generally consistent with those of the literature, differences in stream properties and phase behaviors suggest that optimal process configurations from simulations may have significant uncertainties. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.