Performance tests of a small, high-speed, uniflow steam engine (expander) are described and theoretical and experimental results compared. The engine delivered a maximum of 10.7 hp at 2100 rpm with superheated steam at 230 psi. A minimum steam consumption rate of 23.2 lb/hp-hr was measured at 40% cutoff with 590 F superheated steam. The mechanical efficiency of the engine was determined as 92% and overall engine efficiencies of over 70% were measured. Problems in the engine design uncovered in the tests involving the valve arrangement for short cutoffs are discussed.