An experimental and analytical study of nonequilibrium among the populations of the excited levels of potassium vapor passing through a supersonic nozzle is presented. In the experiment a small amount of potassium vapor was injected into a flow of a mixture of argon and helium, heated by an arc jet electric discharge to a temperature of approximately 2500°K at a pressure of 3 atm. The resulting plasma was passed through a supersonic nozzle (attaining Mach 2.2 at nozzle exit); the gas temperature dropped from 2500° to 1000°K in a time scale of 3 × 10−5sec. The electron temperature (though somewhat independent of the gas temperature) also decreased rapidly, causing the free electrons to recombine with ions of potassium. The recombined electrons passing through the energy level structure of potassium formed the basis of the electronic nonequilibrium. Spectrometric measurements (line intensity and line reversal) of the gas were performed upstream of the nozzle and at the nozzle exit. The electronic populations of expanded potassium vapor were observed to depart from equilibrium in qualitative accordance with theoretical predictions. Larger electron excitation cross sections (than the present theoretical ones) in the energy region near threshold would bring theoretical calculations of excited level populations and electron density at the nozzle exit into better agreement with measured values for these quantities.