A very promising technology to achieve a carbon free energy system is to produce hydrogen from water, rather than from fossil fuels. Iodine-sulfur (IS) thermochemical water decomposition is one promising process. The IS process can be used to efficiently produce hydrogen using the high temperature gas-cooled reactor (HTGR) as the energy source supplying gas at 1000°C. This paper describes that demonstration experiment for hydrogen production was carried out by an IS process at a laboratory scale. The results confirmed the feasibility of the closed-loop operation for recycling all the reactants besides the water, H₂, and O₂. Then the membrane technology was developed to enhance the decomposition efficiency. The maximum attainable one-pass conversion rate of HI exceeds 90% by membrane technology, whereas the equilibrium rate is about 20%.