Water is split into hydrogen and oxygen as the net result of the copper-chlorine (Cu-Cl) thermochemical water decomposition cycle. The cycle involves five steps: (1) HCl(g) production using such equipment as a fluidized bed, (2) oxygen production, (3) copper (Cu) production, (4) drying, and (S) hydrogen production. A chemical reaction takes place in each step, except drying. In the present study the hydrogen production step of the Cu-Cl cycle is assessed thermodynamically using energy and exergy methods and considering relevant chemical reactions. Energy and exergy efficiencies of the H(2) production step are evaluated and parametric studies are carried out on energetic and exergetic aspects considering variable reaction and reference - environment temperatures. At a reaction temperature of 450 degrees C, the reaction heat of the H(2) production step is equal to -55,500 kJ/kmol H(2) (exothermic reaction). At a constant reference-environment temperature of 25 degrees C, the exergy destruction of the H(2) production step varies between 1000 kj/kmol H(2) and 7000 kJ/kmol H(2) when the reaction temperature increases from 300 degrees C to 450 degrees C. The exergy destruction decreases with increasing reaction temperature. At a reaction temperature of 450 degrees C and a reference-environment temperature of 25 degrees C, the exergy efficiency of this step is 99% and decreases with increasing reference-environment temperature and increases with increasing reaction temperature. The hydrogen production process is assumed to be driven by nuclear-based heat, yielding an environmentally benign overall process. 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.