A model is being developed to quantitatively compare and select systems and technology option for defined missions envisioned in the National Aeronautics and Space Administration 's (NASA's) Space Exploration Initiative. This model consists of a modular, top-down hierarchical break-down of the life support system (LSS) into subsystems, and further break-down of subsystems, into functional elements representing individual processing technologies. A series of papers titled Human Life Support During Interplanetary Travel and Domicile has been planned to describe the technique and results. Part I, presented at the 19th ICES Conference, describe the system approach. Part II, presented at this conference, describe Part III, this paper, describes results of a system trade study for a Mars Expedition mission comparing open and closed loop systems. Part IV, also presented at this conference, describes results of trading processing technologies in a closed loop configuration for the Mars Expedition mission.In this paper, the result of configuring a LSS in different ways (totally open loop carbon dioxide removal only, partially closed loop and totally closed loop) are discussed in terms of weight, volume, and power using a baseline the weight vs power trades of the open loop, closed loop and carbon dioxide removal systems. After a detailed presentation of the system trades, preffered choices of an configuration for the Mars Expedition mission are identified.