An engineering evaluation of various process plant designs for hydrogen production by direct thermal decomposition of water using solar energy is carried out mainly with the aim of improving the process efficiency and possibly reducing the production cost. Various process types are considered: a process with hydrogen separation at low temperature, a hybrid version of this process with water electrolysis; a process with hydrogen separation at high temperature; same process with steam cogeneration and/or water electrolysis, and a hybrid version of this process with high-temperature electrolysis. The last two processes are found to be thermodynamically more efficient and the production costs of hydrogen with these processes are relatively low. In the study, the minimum annual plant capacity level is 172 GJ which corresponds to hydrogen production by direct thermal water decomposition only, involving a chemical plant module served by a large dish type solar concentrator unit. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.