A plant was designed that uses a solar sulfur-ammonia thermochemical water-splitting cycle for the production of hydrogen. Hydrogen is useful as a fuel for stationary and mobile fuel cells. The chemical process simulator Aspen Plus® was used to model the plant and conduct simulations. The process utilizes the electrolytic oxidation of aqueous ammonium sulfite in the hydrogen production half cycle and the thermal decomposition of molten potassium pyrosulfate and gaseous sulfur trioxide in the oxygen production half cycle. The reactions are driven using solar thermal energy captured from a heliostat array focused on a receiver. The plant's feed stream is water and the product streams are hydrogen and oxygen; all other materials are contained within the plant. The model is for full-scale operation that would generate 133,333 kg of hydrogen per day, which is equivalent to 370 MW on a lower heating value basis. Thermodynamic properties of chemical species obtained from literature, and from laboratory experiments conducted in another part of this project, were entered into the model to improve its accuracy. Design specifications were placed in strategic areas of the model to aid in its convergence. Model convergence is challenging to obtain because of the many material and energy recycle loops within the plant. Calculator blocks were used to obtain power requirements for the electrolyzer and efficiencies of the entire plant based on definitions from the Department of Energy, which funded this project. Results from this work will aid in the design of a large-scale hydrogen production plant.