CeO 2 was reacted in aqueous SO 2 under SO 2 pressure and steam pressure at 377K to yield a variable composition sulfite-sulfate, containing also water of hydration, admixed with unreacted CeO 2. On heating with I 2 at 745–775K, some of the sulfite was oxidized to sulfate, and hydrolysis was concurrent to yield HI. Yields of HI as large as 47% were measured. S 2 was a concomitance product also, by I 2-catalyzed disproportionation, such that sulfite consumption was 95% for both reactions. A thermochemical cycle for H 2 production utilizing CeO 2 as a recycle reagent is described. Cycle stoichiometry and cycle constraints are discussed. CeO 2 is the end product of the highest temperature reaction, and the enthalpy of oxidation of the by-product S 2 can be utilized in situ at > 1000K to improve thermal efficiency.