A series of alkaline-earth silicate glasses, with compositions ranging from the metasilicate to the ortho- and suborthosilicate, have been synthesized by aerodynamic levitation and CO2 laser melting. They have been studied by high-temperature oxide melt solution calorimetry with 2PbO center dot B2O3 as solvent. The enthalpies of formation from the oxides at room temperature (Delta H-298K,oxides(0,gl)) have been calculated from the solution enthalpies. Glasses in the Ca0.5Mg0.5O-SiO2 system show greater energetic stability than those in the MgO-SiO2 system, with a more pronounced negative enthalpy of mixing near the orthosilicate composition. This stabilization may explain why it is possible to prepare glasses poorer in silica (suborthosilicate) in the Ca0.5Mg0.5O-SiO2 system but not in the MgO-SiO2 system. The thermodynamic observations support earlier structural studies in these systems.