Experimental and theoretical studies were made to provide information on nitrogen oxide concentrations produced by gas turbine engines. Nitric oxide concentrations of from 100 to 350 ppm, adjusted to stoichiometric conditions, were measured in aircraft turbojet engines. Concentrations of less than 50 ppm, similarly adjusted, were measured in a 60 hp industrial gas turbine. Concentrations of about 100 ppm, also adjusted, were measured in a laboratory combustor of a design similar to gas turbine combustors. Carbon monoxide and unburned hydrocarbon concentrations also were determined. Comparison with predicted equilibrium concentrations shows strong departures from equilibrium.