Ab initio G2M(MP2) and CCSD(T)/6-311 +G(3df)//MP2/6-31G(d) calculations show that the Ca + CO2 reaction proceeds by formation of a C-2v-symmetric cyclic CaOCO molecule, similar to1 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 + CO2. The reaction mechanisms of different alkali earth metals with carbon dioxide are compared.