The efficient thermal control of vehicles is essential to the success of every single NASA mission. All vehicles have very tight requirements for the thermal control systems while simultaneously placing incredibly stringent demands upon them. These demands are getting even more intense given the shift towards variable heat rejection, which is essential in missions reaching beyond the lower earth orbit. Specifically, the thermal control fluid must maintain excellent thermal properties for heat rejection under peak conditions while at the same time remain liquid at extremely low temperatures down to -90 Celsius. Currently used fluids either do not meet the low temperature requirement (glycol/water mixture) or do not have thermal properties conducive to a compact, efficient system (Galden). Mainstream has identified several promising next generation thermal fluids using computation chemical techniques. In the Phase I, Mainstream will experimentally evaluate the thermal properties of promising single chemicals and chemical mixtures. These data will be used to determine the overall benefit to a thermal control system in terms of thermal performance and pumping power. In Phase II, Mainstream will perform more long term durability, compatibility and performance studies in a simulated test-loop representative of conditions encountered on NASA spacecraft.