Gas-phase oxidation of CO in the presence of rhenium cations with carbonyl and oxygen ligands has been studied by Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. Rhenium cations have been generated by the electron impact of Re-2 (CO)(10) vapour. Contrary to the unreactive rhenium ions, rhenium monocarbonyl ions have been found to react with O-2 molecules yielding rhenium monoxide ions and CO2 molecules. ReO+ ions are subsequently oxidized with O-2 to di- and trioxide ions. The bond energies in rhenium oxide ions were estimated as D-0(Re+-O)=104+/-14, D-0(ReO+-O)<118, D-0(ReO2+-O)=122+/-4 kcal/mol. Simultaneous addition of CO and O-2 molecules to the reaction volume leads to the gas-phase catalytic oxidation of CO with pairs of rhenium oxide ions ReO3+/ReO2+ serving as the oxidized and reduced forms of the catalyst. The mechanisms of the above reactions are discussed in connection with that for oxidation of CO over solid oxide catalysts.