A technology demonstration Loop Heat Pipe (LHP) has been tested extensively in support of the implementation of this two-phase thermal control technology on NASA's Earth Observing System Tropospheric Emission Spectrometer (TES) instrument. This cryogenic instrument is being developed at the Jet Propulsion Laboratory for NASA. This paper reports on the transient characterization testing results for a propylene LHP. Steady state performance and model correlation results can be found elsewhere .In applications, when a component of large mass on an instrument or spacecraft is attached to a LHP evaporator, there is a concern that the LHP will not start when power is applied to the component until a significant temperature overshoot from the equilibrium temperature is developed. In some space applications, this may be a problem because the maximum allowable flight temperatures (AFTs) may be exceeded. Similarly, when power is removed from the component, there is a concern that the LHP will continue to operate, for some extended period of time, due to the sensible heat available from the large mass. Its important to understand the LHP behavior in such a situation in order to make reliable component temperature predictions for non-operating scenarios and to prevent component temperatures from dropping below the minimum non-operating AFT limits.A test program was developed to characterize the startup and shut-down transient behavior of a propylene LHP with a large mass attached to the evaporator. The LHP was tested over the expected operational temperature and power range during ground test operations and in flight for the TES instrument. In addition, a startup heater was implemented on the LHP evaporator and tested under similar conditions. Transient results show the magnitude of the overshoot with and without the startup heater. Recommendations are made for future applications and additional research on this topic.