The mechanisms of the reactions of W and W + with CO x (x = 1, 2) were studied at the CCSD(T)/[SDD + 6-311G(d)]//B3LYP/[SDD + 6-31G(d)] level of theory. It was shown that the gas-phase reaction of W with CO 2 proceeds with a negligible barrier via an insertion pathway, W( 7S) + CO 2( 1A 1) -W(η 2-OCO)( 6A′) ′ OW(η 1-CO) ( 1A) → WO ( 3Σ +) + CO( 1Σ). This oxidation process is calculated to be exothermic by 32.4 kcal/mol. Possible intermediates of this reaction are the W(η 2-OCO) and OWCO complexes, among which the latter is 37.4 kcal/mol more stable and lies 39.7 and 7.3 kcal/mol lower than the reactants, W( 7S) + CO 2( 1A 1), and the products, WO ( 3Σ +) + CO( 1Σ), respectively. The barrier separating W(η 2-OCO) from OWCO is 8.0 kcal/mol (relative to the W(η 2-OCO) complex), which may be characterized as a W +δ-(CO 2) -δ charge-transfer complex. Ionization of W does not change the character of the reaction of W with CO 2: the reaction of W + with CO 2, like its neutral analog, proceeds via an insertion pathway and leads to oxidation of the W-center. The overall reaction W +( 6D) + CO 2( 1A 1) → W(η 1-OCO) +( 6A) → OW(η 1-CO) +( 4A) → WO +-( 4S +) + CO( 1Σ) is calculated to be exothermic by 25.4 kcal/mol. The cationic reaction proceeds with a somewhat large (9.9 kcal/mol) barrier and produces two intermediates, W(η 1-OCO) +( 6A) and OW(η 1-CO) +-( 4A). Intermediate W(η 1-OCO) +( 6A) is 20.0 kcal/mol less stable than OW(η-CO) +( 4A), and separated from the latter by a 35.2 kcal/mol barrier. Complex W(η 1-OCO) +( 6A) is characterized as an ion-molecular complex type of W +-(CO 2). Gas-phase reactions of M=W/W + with CO lead to the formation of a W-carbonyl complex M(η 1-CO) for both M=W and W +. The C-O insertion product, OMC, lies by 5.2 and 69.3 kcal/mol higher than the corresponding M(η 1-CO) isomer, for M=W and W +, respectively, and is separated from the latter by a large energy barrier.