The reactive stability and energy density of magnesium-manganese oxides for high-temperature thermochemical energy storage have been investigated. Three variations of material with molar ratios of manganese to magnesium of 2/3, 1/1, and 2/1 were prepared using solid-state reaction synthesis and were tested for thermochemical reactive stability and energy storage capability. Results show that oxygen released and absorbed (standard cm3 g−1) by the materials remains unchanged over 20 cycles when cycled between 1200 and 1500 °C under an oxygen partial pressure (PO2) of 0.2 atm, indicating excellent reactive stability at high temperatures. Additional confirmation of reactive stability was obtained through testing over 10 energy storage cycles between 1000 and 1500 °C. The energy density of the material between 1000 and 1500 °C was determined through a combination of acid-solution calorimetry and drop calorimetry. The total volumetric energy densities (chemical, phase change, and sensible) obtained for samples of Mn/Mg of 2/3, 1/1, and 2/1 cycled between 1000 and 1500 °C are 1596, 1626 and 1654 MJ m-3, respectively.