FIR spectroscopy was used to identify CH3ONO and CH3ONO2 as products of the nonthermal plasma treatment of simulated diesel exhaust. This is the first observation of CH3ONO formation in such systems. The yield of CH3ONO relative to CH3ONO2 scaled linearly with the average [NO]/ [NO2] ratio in the system. A plot of [CH3ONO]/[CH3ONO2] versus [NO]/[NO2] gives a slope of 1.81 +/- 0.30. This result is indistinguishable from the literature value of the rate constant ratio k(CH3O + NO)/k(CH3O + NO2) = (2.6 x 10(-11))/ (1.5 x 10(-11)) = 1.73 +/- 0.37. The experimental observations suggest that reactions of CH3O radicals with NO and NO2 are the sources of CH3ONO and CH3ONO2 in such systems. The linear relationship between the yields of CH3ONO and CH3ONO2 provides a means of estimating the yield of these compounds during nonthermal plasma treatment of diesel exhaust.