Differential scanning calorimetry (DSC) and in-situ high-energy X-ray diffraction (HEXRD) techniques were used to investigate the high-temperature reactions of a cathode containing Li1−x VPO4F with materials found in a lithium-ion cell, such as graphite additive in the cathode and the electrolyte. The DSC results indicate that the energy released during the reaction of the cathode containing Li1−x VPO4F with the electrolyte was about 1/3 of that released during a similar reaction of the delithiated, layered cathode containing Li1.1[Ni1/3Mn1/3Co1/3]0.9O2. The in-situ HEXRD results provide evidence that the delithiated material, Li1−x VPO4F, reacts with protons and/or lithium ions found in the reaction mixture at elevated temperature, producing an LiVPO4F-like compound. This reaction is the most likely cause for the low energy release during heating, that is, the LiVPO4F-like material appears to be thermally stable at temperatures up to 400 °C in the presence of LiPF6.