Excitation functions for collision-induced dissociation reactions of CO 3 − and NO 3 − to give O − and the corresponding neutral species have been studied using an in-line tandem mass spectrometer. When these ions were prepared from certain gaseous mixtures, larger cross-sections and lower thresholds were observed for the dissociation processes than those found for the same ions in their apparent ground states. These observations suggest the existence of long-lived excited states of CO 3 −∗ and NO 3 −∗. The heats of formation of these excited ionic states were determined to be −4.8 ± 0.1 and −0.3 ± 0.2 eV for CO 3 −∗ and NO 3 −∗, respectively. Possible implications of these findings with respect to the D -region negative ion reaction scheme are discussed.