The phase stability of binary Fe and Mn olivine materials is extensively studied with temperature-controlled in situ X-ray diffraction (XRD) for various Fe/Mn ratios and state of charges (SOCs). We identify that the thermal behavior of partially charged olivine materials is sensitively affected by the Fe/Mn ratio in the crystal. While Fe-rich binary olivine materials readily formed a solid solution phase of Li_1−yFe_1−xMn_xPO₄ near room temperature or with only slight heating, the Mn-rich binary olivine retained its two-phase characteristic up to ca. 250 °C before decomposition into non-olivine phases. The thermal stability and decomposition mechanism of fully delithiated olivine materials are more sensitively affected by the Fe/Mn ratio in the crystal. The decomposition temperature varies from 200 °C to 500 °C among the different Fe/Mn ratios. It is generally observed that the Mn-rich binary olivine materials are inferior to the Fe-rich ones with respect to the thermal stability in the delithiated state.