NMR spectra of Li-7 nucleus have been measured in LiCoO2 and LiNiO2. Line widths of the spectra decreased with increasing temperature above 400 K. The decrease obviously indicates the motional narrowing originating from diffusing Li+ defects. A simple ion hopping model revealed that the diffusion constant of LiCoO2 with an activation energy of 0.3 eV is larger than that of LiNiO2 with the activation energy of 0.6 eV. This suggests that lithium ions are easier to diffuse in LiCoO2 than in LiNiO2 at high temperatures. (C) 2000 Elsevier Science B.V. All rights reserved.