LiCoO2 and LiMn2O4 cathodes were studied by X-ray diffractometry (XRD) and electron diffraction after ageing in the charged state at elevated temperature. Some cathodes were stopped at different times during ageing and XRD measurements were taken to monitor changes in the crystal structure over ageing time. The results indicate that Li-ions intercalate into the cathodes lattice during ageing thus decreasing the available discharge capacity. Analysis of electron diffraction patterns of LiCoO2 and LiMn2O4 retrieved from the cathodes after ageing shows that irreversible crystallographic transformations have taken place in both electrodes. Dark field imaging illustrates that LiCoO2 forms a layer of spinel phase on its surface. In LiMn2O4 a tetragonal distorted spinel is observed when the cathode has been in the 3V regime for considerable length of time.