The thermodynamic analysis of the first stage of enrichment of heavy water production by the Girdler Sulfide (GS) process is described. Use of the Second Law of Thermodynamics as analysis tool allows a simple and useful procedure for the quantification of the transit energy in the process. Such thermodynamic analysis based on the concepts of irreversible entropy increase has frequently been suggested as pointers to sources of inefficiency in chemical processes. Furthermore, this study points out where the irreversibilities of the process are located, a generalized discussion of the cost with respect to the optimization parameters, and provides a generalized discussion from the successful application of the technique. (C) 2007 American Institute of Chemical Engineers.