Two series of small polyatomic ions, HxCO+ and HxN2+ (x = 1, 2, 3), were systematically characterized using three correlated theoretical techniques:  density functional theory using the B3LYP functional, spin-restricted second-order perturbation theory, and singles + doubles coupled cluster theory with perturbative triples. On the basis of thermodynamic data, the existence of these ions in inductively coupled plasma mass spectrometry (ICP-MS) experiments is not surprising since the ions are predicted to be considerably more stable than their corresponding dissociation products (by 30−170 kcal/mol). While each pair of isoelectronic ions exhibit very similar thermodynamic and kinetic characteristics, there are significant differences within each series. While the mechanism for dissociation of the larger ions occurs through hydrogen abstraction, the triatomic ions (HCO+ and HN2+) appear to dissociate by proton abstraction. These differing mechanisms help to explain large differences in the abundances of HN2+ and HCO+ observed in ICP-MS experiments.