Measurements of velocity, temperature, entrained air, and Ar/He concentration profiles in a subsonic thermal plasma jet were obtained using an enthalpy probe and mass spectrometer combination. The rapid entrainment of air into the jet results in rapidly decreasing velocities and temperatures. A significant diffusive separation of the premixed Ar/He feed gases is observed in the large temperature gradients present. Predictions are obtained from a comprehensive computational model of the thermal plasma jet. The 2-D axisymmetric model includes self-consistent effective binary and ambipolar diffusion and uses a k-epsilon turbulence model.