A sulfur-iodine cycle consists of the following three reactions: 2H 2O + SO 2 + I 2 → H 2SO 4 + 2HI, H 2SO 4 → H 2O + SO 2 + 1 2O 2 , 2 HI → H 2 + I 2. It was found that the first reaction can be performed as a cell reaction without the addition of external energy. The sulfuric acid and the hydriodic acid are produced separately in the anode and cathode compartments, respectively. The second and third reactions can be carried out as catalytic thermal decompositions. A process flow sheet of this cycle and its mass balance was based on experimental results, and the heat balance for this cycle was made. It was found that internal heat exchange for this cycle was very large (about 2600 kcal/mol H 2), due mainly to the low yield of the decomposition reaction of hydrogen iodide. Theoretical and experimental studies were made to improve the yield of this reaction. The following three methods seem to be promising for this purpose: (1) continuous removal of the hydrogen produced in the reaction zone; (2) performance of the reaction at low temperature (185–250°C) and high pressure (100 atml; and (3) substitution of the benzene-cyclohexane cycle (6HI − C 6H 6 → C 6H 12 + 3I 2; C 6H 12 → C 6H 6 − 3H 2) for the hydrogen iodide decomposition step.