In the copper-chlorine (Cu-Cl) thermochemical cycle water is decomposed into its constituents (oxygen and hydrogen) by a series of chemical reactions. The cycle involves five steps in which three thermally driven chemical reactions and one electrochemical reaction take place. Oxygen is produced during one of the main chemical reactions. In the present study, the O-2 production step is described with its operational and environmental conditions, and energy and exergy analyses are performed. The cycle is assumed driven using nuclear energy. Various parametric studies are carried out on energetic and exergetic aspects of the step, considering variable reaction and reference-environment temperatures. At a constant reference-environment temperature of 25 degrees C, the exergy destruction of the O-2 production step varies between 4500 and 23,000 kJ/kmol H-2 when the reaction temperature increases from 450 to 1000 degrees C. At a 500 degrees C reaction temperature and a 25 degrees C reference-environment temperature, the exergy destruction for this step is found to be 5300 kJ/kmol H-2. At a reaction temperature of 500 degrees C and a reference-environment temperature of 25 degrees C, the exergy efficiency of the step is determined to be 96% and to decrease with increasing reaction temperature and/or reference-environment temperature. (C) 2008 Elsevier Ltd. All rights reserved.