Tandem mass spectrometry is used to show that low energy collisions of ionized halocarbenes, including CBr2.+ and CBrCl.+, with molecular oxygen lead to (i) decarbonation with formation of the dihalogen molecular cation and (ii) oxygenolysis yielding BrCO+. Both reactions may occur via the same ion-molecule addition product of molecular oxygen and the ionized carbene. Reaction is favored at low collision energies, such as are encountered in an ion trap. insights into the energetics of the reactions of CBr2.+ with O-2 are obtained from ab initio molecular orbital theory. Other aspects of the positive ion chemistry of dioxygen with various halogen-containing ions are also discussed. Dioxygen activation by halocarbon ions suggests additional channels that might affect the fate of halocarbons and the ozone balance in the atmosphere.