The iodine–sulfur (IS) thermochemical process for hydrogen production is one of the most promising approaches in using high-temperature process heat supplied by a nuclear reactor. This process includes three reactions that form a closed cycle: the Bunsen reaction, in which iodine, water, and sulfur dioxide react to form sulfuric acid and hydriodic acid (HI); HI decomposition; and sulfuric acid decomposition. However, the side reactions between H2SO4 and HI may disturb the operation of the IS closed cycle. For optimal process conditions, the reaction kinetics between H2SO4 and HI should be examined. In this work, a preliminary kinetic study was conducted. Using the initial reaction rate method, the kinetic parameters of the reaction between sulfuric acid and HI, such as the apparent reaction orders and rate constant were determined. For I, the apparent reaction order was approximately 1.77, whereas the orders for H+ and SO42− were 7.78 and 1.29, respectively. The apparent rate constant at 85 ± 1°C was approximately 2.949 × 10−11 min−1 (mol/L)−9.84. The H+ concentration had more significant influence on the reaction rate than those of SO42− and I. Such basic data provide useful information for related process design and further kinetics study.