LiFePO4 has a fully ordered olivine structure in samples prepared by solid state reaction below similar to 800 degrees C but, with increasing temperature, a small amount of Li and Fe site exchange occurs reaching a value of about 4% just below melting at 975 degrees C. The disorder is reversible on annealing at lower temperatures and is detected by changes in lattice parameters and in cation site occupancies obtained by Rietveld refinement of X-ray powder diffraction data. (C) 2011 Elsevier B.V. All rights reserved.