Abstract: Negatively charged organic uranyl species (HOUO₂OH), prepared with uranyl acetate using extractive electrospray ionization multiple-stage tandem mass spectrometry, reacted with oxygen to yield a novel UO₅ anion species in the gas phase. The molecular structure of UO₅ was confirmed by using collision-induced dissociation (CID) mass spectrometry experiments. The effects of experimental parameters on the yield of UO₅ anion were experimentally evaluated. The data have shown that the electron cloud radius (r) of the polyatom anion (e.g., HOUO₂OH) was tunable within the range r_inert≤r_Reactive≤r_{decomposition} by changing the collision energy of CID. This mechanism can be further used to catalyze a certain type of chemical reactions.