Equilibrium conditions of the main reactions of the SI cycle were studied in this paper. The extent of reaction for two main reactions of this cycle including sulfuric acid decomposition and hydrogen iodide decomposition was evaluated as a function of temperature, pressure, and initial mole number of reactants. In the reaction modelling of sulfuric acid and sulfur trioxide decompositions, two models including simple and multiple reaction models were considered. In the simple model, it was assumed that two decomposition reactions proceeded independently. However, in the multiple reaction model, both reactions were considered simultamneously. It was shown that the Bunsen reaction completely proceeded in the range of operating conditions of the Bunsen reactor. Also, sulfuric acid was demonstrated to be decomposed at a higher rate when the reaction temperature and initial concentration of the sulfuric acid and sulfur trioxide increased. Furthermore, the operating pressure had to be maintained at lower values and the generated sulfur dioxide had to be withdrawn from the reactor in order to have a higher yield of product in this reactor. Finally, in the hydrogen iodide decomposition, the operating pressure, temperature, and concentration of hydrogen iodide had to be maintained at higher values to have a complete reaction. Moreover, the generated hydrogen was shown to be withdrawn from the hydrogen iodide reactor to prevent from backward reaction.