An experimental study was made of the performance of Lorentz accelerators. Such accelerators involve the application of electric and magnetic fields to accelerate flows of partially ionized gases. Propulsion efficiencies of the accelerator of up to 28% were obtained indicating the possibility of achieving even higher efficiencies with accelerators of more advanced design. Thrust efficiencies in the vicinity of 90% were achieved; and overall efficiencies of 50% were obtained for the combined performance of the plasma generator and the Lorentz accelerator. Currents up to 1000 amperes and magnetic fields up to 0.205 webers/m2 were used. The optimum magnetic field was found to be approximately 0.180 weber/m2.