Japan Atomic Energy Agency (JAEA) has launched the preliminary design of a hydrogen production system based on Japan's first high-temperature gas-cooled reactor high temperature engineering test reactor (HTTR) since 2005. The selected method of hydrogen production is the thermochemical water-splitting iodine-sulphur (IS) process. The conceptual design of the HTTR-IS system is described in this paper. Using the secondary helium loop of the HTTR for coupling the hydrogen production system to the reactor, the possibility of constructing the IS process as a non-nuclear grade or conventional chemical plant is investigated for the available structure of the HTTR-IS system and the proposed heat mass balance. A hydrogen production rate of about 1100 Nm(3)/h and a thermal efficiency of 44% are shown in the process flowsheet evaluation for the HTTR-IS system. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.