In this study vanadyl, VO²⁺ – halide (fluoride, chloride, bromide, and iodide), cations coordinated by acetonitrile ligands were investigated in the gas phase.  The aim of this experiment was to investigate the relationship between halide ligand polarizablility and the tendency for the vanadyl complex to add O₂ or H₂O via a gas-phase association reaction.  Electrospray ionization (ESI) was used to generate ions of the form [VOX(acn)₂]⁺, and multiple-stage tandem mass spectrometry (MS-MS) and collision induced dissociation (CID) were used to investigate the intrinsic reactions with H₂O and O₂ as well as dissociation reactions of both the precursor and product ions.  We found that hydration of the [VOX(acn)₂]⁺ complexes was dependent on the electronegativity of the halide, and the hydration rates decreased following the trend F > Cl > Br > I.  The addition of O₂ followed the opposite trend.  Addition of H₂O appeared to occur when VO-anion species had a relatively high positive charge, but with increased transfer of charge from halide to VO²⁺, the addition of dioxygen was favored.