Sulfur-Iodine (SI) cycle is one of the leading candidates in thermochemical processes for hydrogen production. In this work SI cycle is simplified into three main reactions -Bunsen reaction, sulfuric acid decomposition and the hydriodic acid decomposition. Each reaction was modeled with single reaction chamber. The simplified model uses basic heat and mass balance for each of the main three reactions. For sulfuric acid decomposition and the hydriodic acid decomposition, the reaction heat, latent heat and the sensible heat were considered. Since Bunsen reaction is exothermic and its overall energy contribution is small, its heat energy is neglected. However the inputs and output streams from the Bunsen reaction are accounted in balancing the total stream mass flow rates from the SI cycle. The heat transfer between the reactor coolant (in this case helium) and the chemical reaction chamber was modeled with transient energy balance equations. A detailed mathematical model was developed for coupled SI cycle to a high temperature gas reactor. The steady-state and transient behavior of the coupled system is studied with the model and the results of the study are presented.