[en] Thermochemical water decomposition has been identified as a potentially significant future process for the production of hydrogen, which is currently an important industrial commodity, and has significant future potential as a fuel. In this paper, the thermodynamic performance of the Ispra Mark-10 thermochemical water decomposition process is investigated. Exergy analysis is used since energy analysis on its own does not pinpoint true process inefficiencies, and often does not provide rational efficiencies. The analysis has indicated that the principal losses occur in the primary water decomposition reactors, and are mainly due to internal irreversibilities associated with chemical reaction and heat transfer across large temperature differences, and that the losses associated with effluents (particularly cooling water) are not that significant