Gas-phase complexes of uranyl(V) ligated to anions X- (X = F, Cl, Br, I, OH, NO3, ClO4, HCO2, CH3CO2, CF3CO2, CH3COS, NCS, N-3), [UO2X2](-), were produced by electrospray ionization and reacted with O-2 in a quadrupole ion trap mass spectrometer to form uranyl(VI) anionic complexes, [UO2X2(O-2)](-), comprising a superoxo ligand. The comparative rates for the oxidation reactions were measured, ranging from relatively fast [UO2(OH)(2)](-) to slow [UO2I2](-). The reaction rates of [UO2X2](-) ions containing polyatomic ligands were significantly faster than those containing the monatomic halogens, which can be attributed to the greater number of vibrational degrees of freedom in the polyatomic ligands to dissipate the energy of the initial O-2-association complexes. The effect of the basicity of the X- ligands was also apparent in the relative rates for O-2 addition, with a general correlation between increasing ligand basicity and O-2-addition efficiency for polyatomic ligands. Collision-induced dissociation of the superoxo complexes showed in all cases loss of O-2 to form the [UO2X2](-) anions, indicating weaker binding of the O-2(-) ligand compared to the X- ligands. Density functional theory computations of the structures and energetics of selected species are in accord with the experimental observations.