An analytical study has been made of the kinetics of formation of NO in hydrocarbon-air combustion. Two UNIVAC 1108 computer programs were utilized to obtain time-dependent concentrations of chemical species in a subsonic stream. Inlet conditions specified were the temperature, pressure, and composition. Inlet tempera-lures were varied from 1000 K to 2000 K at pressures of 1 to 10 atm for equivalence ratios of 0.8 to 1.25. The inlet composition was taken lo be a mixture of non-reacted gases. It was found that equilibrium in the nitrogen oxides is very slowly attained with respect to the carbon and hydrogen oxides. The implication of this result is that observations of NO concentrations well below equilibrium values in certain types of engine exhausts may be correlated with the kinetics of formation, and that combustion temperatures and residence times, rather than exhaust temperatures, determine the level of NO in exhaust gases.