A simple Loop Heat Pipe (LHP) with a single evaporator and condenser was tested and modeled with two different working fluids: ammonia and propylene. While ammonia exhibits many desirable heat transfer characteristics, its freezing point is too high to prevent freezing in the condenser lines during a safing mode on a satellite platform. Consequently, propylene makes a good compromise since it has a lower freezing point and relatively good heat transfer properties.The performance of the LHP with ammonia was characterized by a series of tests with heat loads of 20 to 800 watts placed on the evaporator. With the LHP filled with propylene, it was tested with heat loads of 20 to 200 watts to the evaporator. The sink temperatures on the condenser ranged from −10°C to 20°C. The constant conductance performance of the LHP was 170 W/K with ammonia and 44 W/K with propylene.Steady state performance data of the LHP was used to validate a nodal network model of the device. The evaporator temperature as a function of heat load was compared between the collected data and the model. The average difference between the observed and the predicted evaporator temperatures were 0.85°C with ammonia and 1°C with propylene.