We report a detailed computationally study of the stability of the alkaline earth metal peroxides MO 2 (M ؍ Ba, Sr, Ca, Mg, Be) with respect to decomposition into the corresponding oxides MO and molecular oxygen using Hartree-Fock and density functional theory (DFT) techniques. A comparison between calculated and experimental binding energies indicates that the DFT method is most suitable for a correct description of the peroxide bond. The DFT reaction energies for the peroxide decomposition MO 2 P MO ؉ 1 2 O 2 show that only BaO 2 and SrO 2 are thermodynamically stable compounds, while CaO 2 (in the calcium carbide structure), MgO 2 , and BeO 2 (in the pyrite structure) are energetically unstable with reaction energies of ؊24.7, ؊26.8, and ؊128.7 kJ/mol, respectively, and are therefore unlikely to exist as pure compounds. The published calcium carbide structure for CaO 2 is probably incorrect, at least for pure calcium peroxide, since apart from the thermodynamical instability the compound is more stable in the pyrite structure by 25.5 kJ/mol. Our analysis suggests that the water and/or hydrogen peroxide content of experimentally prepared MgO 2 samples is necessary for the stabilization of the structure, while BeO 2 is clearly unstable under ambient conditions. We studied also the effect of the zero point energies and the entropies on the decomposition free energies and, for this purpose, performed atomistic lattice simulations based on interatomic potentials, which we derived from our ab initio data; the results indicate a negligible effect of the zero point energies, while the entropy terms favor the decomposition reaction by ca. 20 kJ/mol at 298.15 K.