An apparatus to measure the mass specific impulse, Isp, of partially ionized helium flowing in a converging-diverging nozzle, was designed and fabricated. The mass specific impulse increased as the stagnation enthalpy, 110, increased. The effect on the mass specific impulse and the exit enthalpy, h2, of varying nozzle length (and area ratio) was observed. The maximum mass specific impulse measured in this investigation was approximately four times the mass specific impulse of a high performance chemical rocket fuel.