The thermal stability of LiCoPO4 cathodes charged to different lithium contents was studied by synchrotron diffraction and differential thermal analysis. Both olivine-like phases LizCoPO4 (z = 0.6) and CoPO4 appearing during the delithiation of LiCoPO4 are unstable upon heating, and decompose readily in the range 100-200 degrees C. The decomposition of lithium-poor phases leads to gas evolution and the crystallization of Co2P2O7. The role of carbon present in the electrochemically delithiated samples is discussed. The significantly lower stability of charged LiCoPO4 in comparison with LiFePO4 is a serious challenge for the application of this material in rechargeable Li-ion batteries.