Abstract: Hydrogen is produced from water by first reacting I.sub.2, SO.sub.2 and H.sub.2 O to make hydrogen iodide and sulfuric acid. A substantial molar excess of SO.sub.2 and I.sub.2 in the reaction zone creates a lighter sulfuric acid-bearing phase and a heavier polyiodic-acid-bearing phase. The heavier phase is separated, degassed and then contacted with phosphoric acid to permit distillation of HI of low water content and recovery of I.sub.2 as a separate fraction. Hydrogen is recovered from HI vapor, as by thermal decomposition.

Abstract: Hydrogen is thermochemically produced from water in a cycle wherein a first reaction produces hydrogen iodide and H.sub.2 SO.sub.4 by the reaction of iodine, sulfur dioxide and water under conditions which cause two distinct aqueous phases to be formed, i.e., a lighter sulfuric acid-bearing phase and a heavier hydrogen iodide-bearing phase. After separation of the two phases, the heavier phase containing most of the hydrogen iodide is treated, e.g., at a high temperature, to decompose the hydrogen iodide and recover hydrogen and iodine. The H.sub.2 SO.sub.4 is pyrolyzed to recover sulfur dioxide and produce oxygen.