The potential energy surface for the reaction of the CF 3O radicals with CO was investigated. The geometries and vibrational frequencies of the reactants, transition states, intermediates, and products were calculated at the UB3LYP/6-311+G(2d,p), UB3LYP/6-311+G(3df,2p) and UMP2/6-311+G(2d,p) levels of theory. The energies were improved by using the G2M(CC2) and G3B3 methods. The calculation suggests the reaction proceeds via either the fluorine abstraction of CF 3O by CO to produce FCO + CF 2O with a high energy barrier or the barrierless association of the reactants to form the trans-CF 3OCO intermediate. The trans-CF 3OCO is predicted to undergo subsequent isomerization to cis-CF 3OCO or dissociate directly to the products FCO + CF 2O and CF 3 + CO 2. The collisional stabilization of trans-CF 3OCO is dominant at room temperature, while trans-CF 3OCO isomerizing to cis-CF 3OCO followed by dissociating to CF 3 + CO 2 is accessible when temperature rises. The reason for only trans-CF 3OCO without cis-CF 3OCO observable in Ashen’s experiment [S.V. Ahsen, J. Hufen, H. Willner, J.S. Francisco, Chem. Eur. J. 8 (2002) 1189] is cis-CF 3OCO can be produced only via the isomerization of trans-CF 3OCO, and its yield is inappreciable at a low experimental temperature. The enthalpies of formation for the two conformations of CF 3OCO have been deduced: Δ f H 0 ∘ ( trans-CF 3OCO) = −196.25 kcal mol −1, Δ f H 298.15 ∘ ( trans-CF 3OCO) = −197.46 kcal mol −1, Δ f H 0 ∘ ( cis-CF 3OCO) = −193.64 kcal mol −1, and Δ f H 298.15 ∘ ( cis-CF 3OCO) = −194.90 kcal mol −1.