The objective of this paper is to present an estimate of the second law thermodynamic efficiencies of the various units comprising an Environmental Control and Life Support Systems(ECLSS). The technique adopted here is based on an evaluation of the ‘lost work’ within each functional unit of the subsystem. Pertinent information for our analysis is obtained from a user interactive integrated model of an ECLSS. The model was developed using a chemical process simulator called ASPEN (Advanced System for Process Enginering). A potential benefit of this analysis is the identification of subsystems with high entropy generation as the most likely candidates for engineering improvements.