The purpose of this paper is to discuss the selection of a low-temperature coolant used to support Space Station Freedom Active Thermal Control System (SSF ATCS) qualification testing. SSF ATCS testing will use liquid nitrogen (LN2) to reduce the temperature of an intermediate coolant to as low as −78°C; the coolant will then interface with the ATCS working fluid to simulate the heat sink conditions of space. Selection of the intermediate coolant required investigation of the following coolant properties: low freezing point, low viscosity at low temperatures, very low to no toxicity and flammability, and low to moderate vapor pressure at room temperature. Of the four refrigerants that were initially considered, R-124 appeared to be the most attractive. A test was performed to verify the freezing point of R-124. The R-124 test results and a comparison of the four refrigerants are described in detail in this paper. Functional descriptions of the coolant thermal loop, the flow schematic, and the heat exchangers for NH3/R-124 and R-124/LN2 are also described.