An ammonia-based thermochemical energy storage system is under investigation at the Australian National University. In recent years, one line of research has been the analysis of ammonia synthesis reactors. A two-dimensional numerical reactor model was previously validated with experimental results and was used in this study to compare maximum thermal output with maximum exergy output of the reactor investigated. The generic concept of exergy analysis is explained and some thermodynamic background is given. This study shows that a different set of 'optimum' reactor conditions results if optimized for maximum exergy output rather than maximum thermal output. Exergy analysis proved to be a useful technique for gathering information about the energy system investigated that would with an energy analysis purely based on first-law analysis not have been accomplished. Results tend to favour either very small diameter reactors to get close to the maximum rate curve or adiabatic reactors as used in industry. (C) 2002 Elsevier Science Ltd. All rights reserved.