Mass spectrometric experiments at extremely low (< 10(-6) mbar) and moderate (0.5 mbar) pressures are used to examine the reactions of atomic vanadium cation with molecular oxygen and water. With Oz, rapid O-atom abstraction gives rise to the formation of VO+ cation (k = 3 x 10(-10) cm(3) molecule(-1) s(-1)). Interestingly, despite a similar thermochemistry, the O-atom transfer from water to bare V+ is less efficient by more than an order of magnitude (k = 8 x 10(-12) cm(3) molecule(-1) s(-1)). Subsequent associations of VO+ with either Oz or H2O occur with very low efficiencies and involve termolecular stabilization mechanisms. The low probability of degenerate O-16/O-18 exchange between VO+ and water indicates the operation of a sizable kinetic barrier. Ab initio calculations using density functional theory lend further support to the interpretation of the experimental data and provide the first thermochemical information on VOn+ cations with n > 2, as well as some hydrated species. In general, the dipolar water ligand is found to be much more strongly bound to the cationic vanadium complexes than is dioxygen.