Helium was expended through a supersonic nozzle in a wind tunnel after being partially ionized by an electric arc. Calculations based on the stagnation proportion of the gas end the area ratio of the nozzle indicated that the static pressure in the wind tunnel was too high to allow isentropic expansion of the gas. A previous investigation noted an extreme reduction of thrust output at this area ratio, as compared to the thrust obtained at smaller area ratios, indicating the possibility of a chock in the nozzle. Probes wore developed to measure the degree of ionization and pressure downstream of a normal shock. From these two properties, the free stream properties of the gas were calculated. The Mach number was far below that predicted for isentropic expansion, indicating that a shock did occur in the nozzle.