The existing Shuttle Extravehicular Mobility Unit (EMU) has served NASA well for sometime, however, it uses a large amount of consumables including water, O2 and lithium hydroxide. In order for extended missions to the Moon and Mars to be successful, two new portable life support systems (PLSS) designs have been proposed that will minimize the amount of consumables used and will be more reliable due to simplified designs. This paper considers one such PLSS, currently designated the Cryogenic-PLSS (CPLSS). The reason for this designation is because it uses liquid O2 to provide the breathing gas for the astronaut and to provide backup cooling for the astronaut. In order to understand how the system will function in space and to evaluate final design parameters, a transient thermal model has been developed using the software package MATLAB/Simulink. In addition to the components of the CPLSS, a simplified suit model, and a human thermal model are included to gain insight into the interactions between the CPLSS, the human, and the environment. This paper will cover the basic operation and theory behind the model, as well as initial performance studies, and will discuss design considerations for the new CPLSS.