An upper bound and a best estimate of the efficiency of a thermochemical iodine sulphur water splitting cycle coupled with a High Temperature Reactor operating between 760 K and 1144 K are proposed in this publication. An upper bound of 51% is found assuming ideal reversible chemical reactions and constraints related to the heat exchange between helium and chemical process. It appears that the separations between water and sulphuric acid on one hand and water, HI and iodine on the other hand are the most energy consuming. The best estimate of the efficiency is found between 33% and 36% according to the assumptions made for the temperature pinch at the heat exchangers of the hydrogen production section. More realistic values can be obtained after having designed the heat exchangers of this section. (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.