The hydriodic acid decomposition section (HIx section) of the iodine-sulfur water splitting hydrogen production process (I-S process), to a large extent, determines the efficiency of the whole process. The flowsheet of the HIx section includes several internal circuits, which can recover the lost HI due to the distillation of the azeotrope and the low one-pass conversion ratio of the HI decompostion reaction. The employment of the circuits is capable of increasing the utilization of the input HI. However, different designs of the circuits influence the mass and heat balance of the flowsheet, and furthermore result in distinctions of the hydrogen production efficiency and the required input rate. In order to examine the performance of the HIx section flowsheets with diverse internal circuits and pursue high efficiency, the present research used simulation methods to invesigate the HIx section flowsheet. The effects of different designs of internal circuits in the HIx section on the required input flow rate and heat duty for producing 1 mol hydrogen were studied. Based on the results, the most efficient circuit design was proposed and compared with the work of other researchers. In addition, the heat recovery problems of the flowsheet were discussed. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.