The distribution of Fe3+ and Ni3+ impurities and the electrochemical performance of LiCoO2 electrodes were examined. Commercial LiCoO2 powders supplied by Aldrich were used. The electrochemical performance of LiCoO2 was modified by rotor blade grinding of LiCoO2 followed by thermal treatment. Structural information on Fe3+ and Ni3+ impurities was obtained using both conventional X-band and high-frequency electron paramagnetic resonance spectroscopy (EPR). It was found that Fe3+ occupies a Co-site having a higher extent of rhombic distortion, while Ni3+ is in a trigonally distorted site. After rotor blade grinding of LiCoO2, isolated Fe3+ ions display a tendency to form clusters, while isolated Ni3+ ions remain intact. Re-annealing of ground LiCoO2 at 850°C leads to disappearance of iron clusters; isolated Fe3+ ions are recovered. The electrochemical performance of LiCoO2 was discussed on the basis of isolated and clustered ions.