Carbon-coated LiMn x Fe1−x PO4 (x = 0, 0.5 and 1) is fully delithiated using NO2BF4 oxidant to investigate thermal behavior as a function of temperature as high as 700 °C in an Ar atmosphere. Reitveld refinement of X-ray diffraction (XRD) studies indicates readiness of chemical delithiation, except for Li0MnPO4, hydrated to amorphous MnPO4·2H2O. Thermal studies coupled with thermogravimetric analysis and in situ high temperature XRD demonstrate that the phase stability is affected by substituted Mn at an elevated temperature. The Li0FePO4 is stable up to 500 °C, maintaining the original olivine structure with a small amount of oxygen release. At higher temperatures, the phase was transformed to Fe3(PO4)2 and Fe2P2O7. In contrast, the phase transformation temperature is apparently reduced to as low as 200 °C by the addition of Mn into the crystal structure, which is related to the phase transformation to (Mn0.5Fe0.5)3(PO4)2 associated with fast oxygen loss from the original structure. As a result, the better thermal stability of Li0FePO4 is attributed to less oxygen loss, which delays the phase transformation, resulting in less exothermic heat in the temperature range.