The coupled dynamic and kinetic evolution of chemical species are considered for an assumed exhaust plume from an SST at proposed flight altitudes of about 20 km. A systematic effort is made to determine whether chemical and radiative interactions seriously alter the exhaust specie distributions determined on the basis of scale-dependent diffusion alone. In the present coupled treatment, use is made of the plume dispersion coefficients from previous dynamic calculations, and a moderately complex kinetic mechanism is incorporated. The sensitivity to transport rates is presented, and the role of inhomogeneity correlations is discussed.