O-2 and CO2 do not react under ordinary conditions because of the thermodynamic stability of CO2 and the large activation energy required for multiple double-bond cleavage. We present evidence for a gas-phase O-atom exchange reaction between neutral O-2 and CO2 at elevated collision energies (similar to 160 kcal mol(-1)) from crossed-molecular-beam experiments. CCSD(T)/aug-cc-pVTZ calculations demonstrate that isotope exchange can occur on the ground triplet potential energy surface through a short-lived CO4 intermediate that isomerizes via a symmetric CO4 transition state containing a bridging oxygen atom. We propose a plausible adiabatic mechanism for this reaction supported by additional spin-density calculations.