200-600 ppm of CO inhibit NO conversion in nonthermal Ar plasma, but do not produce N2O. However, 1.01% of CO has no effect on NO conversion, but produces N2O. In general, N2O conversion in Ar plasma decreases with increasing CO concentration. These experimental results cannot be explained by charge transfer reactions of Ar+. Selectivity analysis of all excited states of Ar possibly contributing to NOX conversion without and with CO suggests that only AR(3P2) contributes to NOX conversion and CO dissociation. A kinetic model of 43 reactions is required to model NO Conversion or N2O conversion in Ar without CO, whereas 81 reactions are required to model NO conversion and N2O conversion in Ar with CO. At constant gas pressure, a single set of model parameters can predict NO conversion or N2O conversion without and with CO. All experimental results can be explained using a reaction mechanism in which excited neutral states of Ar are the only active species, which supports the conclusion that cations have a negligible impact on these nonthermal plasma reactions.