Manganese oxides occur widely in nature and have technical applications in various areas. This study quantitatively evaluates the thermodynamic properties of Mn5O8, a binary manganese oxide that has a layered structure and contains coexisting divalent and tetravalent manganese. Three samples of the Mn5O8 phase with slightly different manganese average oxidation states (Mn AOS) were synthesized using a wet chemical method and annealing. Synchrotron X-ray analysis revealed that the samples contain a small amount of a secondary MnO2 phase that cannot be identified using laboratory X-ray diffraction. High-temperature oxide melt solution calorimetry in molten sodium molybdate at 700 degrees C showed that all three samples are slightly higher in enthalpy than an isochemical mixture of bixbyite (Mn2O3) and pyrolusite (MnO2), probably rendering them metastable in free energy with respect to isochemical mixtures of bixbyite and pyrolusite. However, the energetic metastability (endothermic enthalpy) of Mn5O8 is very small (<6 kJ/mol) and does not depend significantly on the Mn AOS. Thus, although Mn5O8 probably does not appear on the equilibrium Mn-O phase diagram, its small metastability allows its synthesis by a variety of low temperature reactions.