Hydrogen production from water using the sulfur-iodine (S-I) thermochemical cycle, powered by combined solar and fossil heat sources, has been investigated. This combined energy supply was conceived in order to operate the chemical process continuously: a solar concentrator plant with a large-scale heat storage supplies thermal load for services at medium temperatures (< 550 degrees C), while a fossil fuel furnace provides heat load at higher temperatures. Additionally, a methanol production plant fed with the carbon oxides generated from fossil fuel combustion was included. Since the sulfuric acid concentration/decomposition section is interfaced to both the fossil furnace and the solar plant, it was studied more exhaustively. Results obtained show that the major part of the total energy demand (ca. 70%) is renewable. An industrial scale plant with hydrogen capacity of ca. 26,000 tons/year coupled with a 267 MWth solar plant was considered, and the specific cost of the produced hydrogen and methanol determined. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.