A test method to measure in situ refrigerant-flow leakage rates in heat pump reversing valves is described. The method is applied to measure the leakage rates of four different valve models under different sets of Operating conditions. The highest leaker exhibited leakage rates in less than 1/4 of 1 percent of the total system mass flow rate. This rate is about 20 times less than that inferred from other test data in the open literature. The measurements reported are limited to a fixed slide position.