The adsorption properties of Cl2 molecules adsorbed on Li and Na atoms supported on various sites of MgO and CaO (100) surfaces have been examined at the DFT/B3LYP level of theory using the embedded cluster model. By comparison, the adsorption on the clean surfaces of the support has been investigated. Our calculations predict weakening of the Cl2 bond due to the presence of the alkali metals, reflected by an elongation of the Cl2 bond by 0.64 Å. Although the regular and defective sites are stronger binding sites in CaO (001) than in MgO (001), the interaction energy decreases as the atomic number of alkali atoms (Li and Na) increases. Metal atoms bind more strongly on the defect sites. The MgO and CaO (001) surfaces not only serve as the supports of Li and Na atoms but also participate in the interaction with Cl2 molecule. Charge transfer takes place from the oxide surface to the adsorbed metal.