The thermal stability of LiCoMnO4 and LiCoMnO4 with 1 wt.-% LiF as an additive was compared upon heating in air. The stoichiometries of samples heat-treated at 700, 800, 900 and 1000 degrees C were analyzed by means of inductively coupled plasma optical emission spectroscopy for cation contents, inert gas fusion analysis for oxygen, and nuclear reaction analysis for fluorine contents, revealing that the sample chemistries remain constant except for oxygen. The oxygen content of the samples correlates with the Li2MnO3 secondary phase fraction, which forms upon heating, as is shown by X-ray powder diffraction. For each temperature, a greater amount of Li2MnO3 is formed in the samples with LiF addition than in the single LiCoMnO4 samples. In situ high-temperature X-ray powder diffraction between 650 and 950 degrees C demonstrates that LiCoMnO4 with LiF addition exhibits the same kind of decomposition reactions upon heating and cooling as the sample without LiF addition. However, for the samples with LiF addition, the formation of the decomposition product, Li2MnO3, is accelerated due to the excess Li through LiF addition, i.e. the stability of the spinel phase decreases due to the supply of lithium. Since lithium is consumed by Li2MnO3 formation and fluorine remains in the samples, we can also conclude that the fluorine anion is incorporated into the spinet or Li2MnO3 phase upon heating LiCoMnO4 and LiF together.