Manganese oxides are abundant in the environment and are essential to various technologies. The four most common binary manganese oxides are MnO2 (pyrolusite), Mn2O3 (bixbyite), Mn3O4 (hausmannite) and MnO (manganosite). In 1965, Oswald et al. reported a new binary manganese oxide with the nominal stoichiometry Mn5O8 or Mn(II)2Mn(IV)3O8. Mn5O8 exhibits a unique monoclinic layered crystal structure. Subsequent studies revealed that the divalent manganese ions in Mn5O8 can be replaced by other divalent metal cations, including Ca2+, Cu2+, and Cd2+ ions, which results in ternary oxides that bear the same crystal structure. In this dissertation, samples of Mn5O8, Ca2Mn3O8, Cd2Mn3O8, Cu2Mn3O8 and a 50-50 solid solution of Ca2Mn3O8-Cd2Mn3O8 have been synthesized to study their thermodynamic properties. Their structures have been confirmed by X-ray diffraction and compositions by iodometric titration and/or electron microprobe analyses. High temperature oxide melt solution calorimetry has been carried out on all samples. Their formation enthalpies from binary oxide end members have been calculated from measured and literature data. The results are discussed in terms of stability, structure, bonding, and significance. Mn5O8 exhibits a slightly endothermic (< 6 kJ/mol) enthalpy of formation from an isochemical mixture of bixbyite and pyrolusite, suggesting that it is energetically metastable. Given that a solid-state reaction involving no gas species usually has very small change in entropy due to the small differences in heat capacities, the free energy of formation of Mn5O8 from an isochemical mixture of bixbyite and pyrolusite is likely to be slightly endothermic or close to zero. Therefore, Mn5O8 is probably a metastable phase not found on the equilibrium Mn - O phase diagram. The energetic stabilities of the ternary M2Mn3O8 samples depend on the M2+ cation (M = Ca, Cd, Cu) in the structure. The enthalpies of formation of M2Mn3O8 from MO and MnO2 are -133.25 ± 4.87 kJ/mol, -50.86 ± 3.52 kJ/mol and +35.42 ± 3.70 kJ/mol for M = Ca, Cd and Cu respectively, suggesting energetic stability decreases in the order Ca, Cd, Mn, and Cu. The same trend is observed in their thermal decomposition behavior, where Ca2Mn3O8 is the most refractory and does not decompose until over 900 °C while Cu2Mn3O8 decomposes well below 500 °C. The 50-50 solid solution of Ca2Mn3O8-Cd2Mn3O8 or CaCdMn3O8 has an enthalpy of formation from CaO, CdO and MnO2 of -117.6 ± 4.75 kJ/mol. It is noticeably energetically more stable than a 1:1 mechanical mixture of Ca2Mn3O8 and Cd2Mn3O8, whose enthalpy of formation from CaO, CdO and MnO2 would be -96.09 ± 3.01 kJ/mol. The enthalpy of mixing for this solid solution is hence -22.14 ± 3.78 kJ/mol, indicating energetically favorable mixing, possibly related to some sort of short range ordering of cations. The results of this dissertation work can aid in future computational and experimental studies on M2Mn3O8 materials. The quantitative study of thermodynamic properties can aid in assessing their stabilities in the environment which will be of interest to heavy/toxic metal immobilization. Also, the choice of materials for applications in areas such as electrochemical energy conversion and storage can directly benefit from these well documented thermodynamic data.