The efficiency of a flashing flow converging-diverging nozzle was measured. The primary motivation for measurement of nozzle efficiency in this work was to improve the performance of an ejector as a refrigerant expander; previous experimentation has suggested that motive nozzle efficiency is poor. Measurement of the efficiency was performed using a technique which does not require knowledge of the non-equilibrium nozzle outlet state. This technique is based on the measurement of nozzle thrust and mass flow rate to determine the actual kinetic energy at the nozzle exit. A vapor compression refrigeration cycle using refrigerant R-12 was employed to test the nozzle. Use of a bubble seeding device upstream of the nozzle to provide nucleation sites in the flow was expected to decrease the non-equilibrium, therefore producing an increase in nozzle efficiency. Experiments were performed at various condenser pressures and mass flow rates. In addition to the efficiency measurement, a parameter to quantify the metastability in the nozzle flow was defined and empirical correlations were developed for both. The experimentation illustrated that the efficiency did indeed increase when flow conditions were closer to equilibrium. Efficiency and metastability parameter results indicate an unexplained dependence on the condenser subcooling. Recommendations were therefore made to investigate this unexpected phenomenon. / M.S.