The energetics of a set of natural and synthetic manganese dioxides having a framework or a layer structure were determined by high-temperature solution calorimetry using sodium molybdate as a solvent. Enthalpies of formation of the metastable manganese dioxides were calculated from the measured values of the enthalpies of drop solution. The stability of the open structure depends mostly on the nature of and the amount of the tunnel (or interlayer) cation and on the degree of hydration. It depends less strongly on the topology of the structure itself and on the average oxidation state of manganese. The synthesis of these metastable microporous materials is not limited by the energetics of the structure.