The remarkable thermal stability of LiFePO4 and its charged counterpart, FePO4, have been instrumental in its commercialization as a lithium-ion battery cathode material. Despite the similarity in composition and structure, and despite the high thermal stability of the parent compound, LiMnPO4, we find that the delithiated phase Li y MnPO4 (which contains a small amount of residual lithium), is relatively unstable and reactive toward a lithium-ion electrolyte. The onset temperature for heat evolution in the presence of 1M LiPF6 in 1:1 ethylene carbonate/propylene carbonate is around 150°C, and the total evolved heat is 884Jg−1, comparable to that produced under similar conditions by charged LiCoO2 electrodes.