The sulfur-iodine thermochemical water-splitting cycle (S-I cycle) is one of the most promising technologies for mass H(2) production. The S-I cycle is generally divided into three sections, one of which involves a H(2)SO(4) concentration and decomposition. In the sulfuric acid processing section (Section 2), H(2)SO(4) is decomposed into H(2)O and SO(3), and then the produced SO(3) is further decomposed into SO(2) and O(2), which takes place in a H(2)SO(4) decomposer and a SO(3) decomposer, respectively. The SO(3) decomposition requires heat of a high temperature and this suggests a heat-exchanger type reactor. To understand the temperature profiles and chemical reactions through a SO(3) decomposer, a dynamic model was developed by considering the heat and material balances in partial differential forms. A model was used to size the decomposer to a proposed design basis and it was also applied to simulate the responses corresponding to the changes of the operation conditions such as increased or decreased flow rates. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.