The mechanisms of the reactions of W and W+ with COx (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 CO2 proceeds with a negligible barrier via an insertion pathway, W(S-7) + CO2((1)A(1)) -> W(eta(2)-OCO)((6)A') -> OW(eta(1)-CO)((1)A) -> WO ((3)Sigma(+)) + CO((1)Sigma). This oxidation process is calculated to be exothermic by 32.4 kcal/mol. Possible intermediates of this reaction are the W(eta(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(S-7) + CO2((1)A(1)), and the products, WO ((3)Sigma(+)) + CO((1)Sigma), respectively. The barrier separating W(eta(2)-OCO) from OWCO is 8.0 kcal/mol (relative to the W(eta(2)-OCO) complex), which may be characterized as a W+delta-(CO2)(-delta) charge-transfer complex. Ionization of W does not change the character of the reaction of W with CO2: the reaction of W+ with CO2, like its neutral analog, proceeds via an insertion pathway and leads to oxidation of the W-center. The overall reaction W+(D-6) + CO2((1)A(1)) -> W(eta(1)-OCO)(+)((6)A) -> OW(eta(1)-CO)(+)((4)A) -> WO+((4)Sigma(+)) + CO((1)Sigma) 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(eta(1)-OCO)(+)((6)A) and OW(eta(1)-CO)(+)((4)A). Intermediate W(eta(1)-OCO)(+)((6)A) is 20.0 kcal/mol less stable than OW(eta(1)-CO)(+)((4)A), and separated from the latter by a 35.2 kcal/mol barrier. Complex W(eta(1)-OCO)(+)((6)A) is characterized as an ion-molecular complex type of W+-(CO2). Gas-phase reactions of MW/W+ with CO lead to the formation of a W-carbonyl complex M(eta(1)-CO) for both MW and W+. The C-O insertion product, OMC, lies by 5.2 and 69.3 kcal/mol higher than the corresponding M(eta(1)-CO) isomer, for MW and W+, respectively, and is separated from the latter by a large energy barrier.