The Hybrid Sulfur Process, as well as similar sulfur cycles for the production of nuclear hydrogen, requires the decomposition of sulfuric acid into sulfur dioxide, oxygen, and water at temperatures above 800 degrees C and at pressures up to 9 MPa. The design of a reactor for this process presents numerous challenges in terms of maintaining small pressure differentials and utilizing currently available materials of construction. This paper focuses on design calculations for a composite reactor that preheats, concentrates, and decomposes sulfuric acid for use in the production of hydrogen. The decomposition reaction takes place within individual tubes of a multitube reactor. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.