The sulfur-iodine (SI) cycle is one of the leading candidates in thermochemical processes for hydrogen production. In this paper a simplified model for the SI cycle is developed with chemical kinetics models of the three main SI reactions: the Bunsen reaction, sulfuric acid decomposition, and hydriodic acid decomposition. Each reaction was modeled with a single control volume reaction chamber. The simplified model uses basic heat and mass balance for each of the main three reactions. For sulfuric acid decomposition and hydriodic acid decomposition, reaction heat, latent heat, and sensible heat were considered. Since the Bunsen reaction is exothermic and its overall energy contribution is small, its heat energy is neglected. However, the input and output streams from the Bunsen reaction are accounted for 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. The steady-state and transient behavior of the coupled system is studied with the model, and the results of the study are presented. It was determined from the study that the hydriodic acid decomposition step is the rate-limiting step of the entire SI cycle.