The energetics of 14 synthetic manganese dioxides, including framework phases with todorokite structure and layer phases with birnessite and buserite structures, were studied using high-temperature solution calorimetry in molten 3Na(2)O(4). MoO3 at 977 K. The composition of these phases with a general formula of NaxMgyMnO2. nH(2)O was systematically varied. Using the measured enthalpies of drop solution, the enthalpies of formation from oxides based on 1 mol of NaxMgyMnO2. nH(2)O at 298 K, Delta H-f((oxides))o, were obtained. The Delta H-f((oxides))o data show no obvious correlation with either content of charge-balancing cations or content of water for the layer and framework phases. Using a new parameter, the hydration number of the charge-balancing cations (N-(hyd) = the number of water molecules per cation), all of the Delta H-f((oxides))o data can be linearly correlated. The linear correlations are Delta H-f((oxides))o = 16.5N((hyd)) - 73.3 for the layer phases, and Delta H-f((oxides))o = 11.7N((hyd)) -64.4 for the framework phases. The average N-(hyd) values for the birnessite, buserite, and todorokite phases are 2, 4, and 2, respectively.