This paper reports the results of the most complete conceptual study conducted to date on hydrogen production using the hybrid chlorine cycle. Three alternative process flow sheets were developed, each capable of producing hydrogen at 35 degrees C (308 K) and 21 bar. The alternative approaches differ primarily in the way HCl is isolated and converted to hydrogen and chlorine gases. Aspen Plus(TM) simulation software was used to model the unit processes, supplemented where necessary by custom Excel spreadsheets. Major equipment was sized for a 200-million kg/yr plant; feasible materials of construction were identified; fixed capital investments and variable costs were estimated. Estimated net thermal efficiencies of the flow sheets range from 30% to 36%, based on the lower heating value of the hydrogen produced. With electrical power valued at $0.05/kWh, the cost of hydrogen produced by the hybrid chlorine cycle would be at least $3/kg. These results indicate that direct electrolysis of water is a more attractive way to produce hydrogen than any presently conceived version of the hybrid chlorine cycle. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.