The Air Force Research Laboratory (AFRL), in cooperation with the University of Dayton Research Institute (UDRI) and Fairchild Controls Corporation, is operating an in-house advanced vapor compression refrigeration cycle system (VCS) test rig known as ToTEMS (Two-Phase Thermal Energy Management System). This test rig is dedicated to the study and development of VCS control and operation in support of the Energy Optimized Aircraft (EOA) initiative and the Integrated Vehicle ENergy Technology (INVENT) program. Previous papers on ToTEMS have discussed the hardware setup and some of the preliminary data collected from the system, as well as the first steps towards developing an optimum-seeking control scheme. A key goal of the ToTEMS program is to reduce the risk associated with operating VCS in the dynamic aircraft environment. One of the key questions regarding the operability of VCS in aircraft which will be addressed is the in-situ measurement of refrigerant charge within the VCS system. Several potential methods of determining whether an appropriate charge of refrigerant exists will be discussed. An appropriate charge level is one which enables safe and efficient operation of the VCS over its designed operating envelope. The implications of these charge states, as applied to both static and dynamic VCS, will be addressed. To be effective for determining whether appropriate charge exists in an aircraft VCS, any potential method will need to operate in real-time and utilize in-situ sensors with current and recorded data, as opposed to relying on external gauges or specialty instrumentation that might be connected during scheduled maintenance. The method chosen may also indicate the minimum sensor suite needed for an aircraft VCS. Preliminary data will be discussed to illustrate the effect of charge level on VCS performance.