An experimental program was carried out to investigate the flow properties of partly ionized monatomic gases in the converging section of a nozzle. Choked flow was maintained at the throat, and the stagnation pressure, mass flow rate, and thrust at the throat were measured. The thrust at the throat is essentially a function of the stagnation pressure. Deviations from the predicted values were interpreted as representing losses due to heat transfer, friction, and turbulence. The thrust data was compared with the values predicted by a one-dimensional isentropic flow theory for partly ionized gases expanding in thermal equilibrium. The theory, presented in terms of dimensionless flow parameters, was valid for any monatomic gas. Data obtained, when plotted in dimensionless form, showed the same deviation from the idealized relationships for both helium and argon.