The widespread application of oxovanadium ions in catalysis has prompted gas-phase model studies of these species by several research groups, including ours. In the present study we are employing electrospray ionization and mass spectrometry as well as electronic structure calculations to explore the addition and ligand exchange reactions of [VOX(CH₃CN)_n]⁺ with H₂O and O₂. Here X = F^–, Cl^–, Br^–, I^–, OH^– and n = 1, 2. This talk will focus on the ab initio calculations we are carrying out to provide information on precursor and product ion structures, reaction thermochemistry, and viable reaction pathways for these reactions. Among the questions of interest in this study are the following. 1) Does the preference for H₂O versus O₂ addition correlate with the electron density on the metal center? 2) Why is addition of H₂O but not O₂ observed for some of the more highly coordinated complexes? 3) Why is conversion of ligated VOX⁺ to VOOH⁺ observed only for X = F^–?