In many hypersonic flows, the excited states of some chemical species are the dominant radiators. Often, these electronically excited particles are present in small concentrations and the chemical kinetics mechanisms for their formation is very complicated and costly to implement in a fully-coupled computational fluid dynamics method. This paper presents an efficient computational method for the calculation of chemical species present in trace amounts, including excited state species. The usefulness of this overlay method is demonstrated with several examples: OH(A) formation from water vapor in a hypersonic shocklayer, state-specific CO vibrational relaxation and CO(a) formation in a rocket motor nozzle, and CO(a) kinetics in an expanding flow. The overlay calculations allow a careful sensitivity analysis of the kinetics modeling and permit comparison with radiance data obtained in-flight.