Gas-phase complexes containing dioxouranium(V) cations ([UO2](+)) ligated with two or three sigma-donating acetone ligands reacted with dioxygen to form [UO2(A)(2,3)(O-2)](+), where A is acetone. Collision-induced dissociation studies of [UO2(A)(3)(O-2)](+) showed initial loss of acetone, followed by elimination of O-2, which suggested that O-2 was bound more strongly than the third acetone ligand, but less strongly than the second, Similar behavior was observed for complexes in which water was substituted for acetone. Binding of dioxygen to [UO2](+) containing zero, one, or four ligands did not occur, nor did it occur for analogous ligated U(IV)O-2 or U(VI)O-2 ions. For example, only addition of acetone and/or H2O occurred for the U(VI) species [UO2OH]+, with the ligand addition cascade terminating in formation of [UO2OH(A)(3)](+). Similarly, the U(IV) species [UOOH](+) added donor ligands, which produced the mixed-ligand complex [UOOH(A)(3)(H2O)](+) as the preferred product at the longest reaction times accessible. Since dioxygen normally functions as an electron acceptor, an alternative mode for binding dioxygen to the cationic U(V)O-2 center is indicated that is dependent on the presence of an unpaired electron and donor ligands in the uranyl valence orbitals.