Ab initio G2M(MP2) and CCSD(T)/6-311+G(3df)//MP2/6-31G(d) calculations show that the Ca+CO 2 reaction proceeds by formation of a C 2 v -symmetric cyclic CaOCO molecule, ~1 kcal/mol below the reactants, with the initial reaction step barrier of about 13 kcal/mol. Then, CaOCO dissociates to the CaO+CO products without exit barrier. The total reaction endothermicity is calculated as 35.4 and 40.4 kcal/mol at the G2M(MP2) and CCSD(T)/6-311+G(3df)//QCISD/6-311G(d) levels, respectively. The reverse CaO+CO reaction is predicted to occur without barrier and can rapidly produce Ca+CO 2 . The reaction mechanisms of different alkali earth metals with carbon dioxide are compared.