X-ray and neutron powder diffraction studies have been made of the single-phase systems LiCoxFe1-xPO4 (x = 0, 0.25, 0.40, 0.60 and 0. 75) to establish how Co2+ substitutes into the LiFePO4 olivine structure. Rietveld refinement shows that all four substituted materials have the same olivine structure (space group: Pnma) with lithium occupying octahedral (4a) sites, and Co2+ replacing Fe2+ at the octahedral (4c) sites. The a and b cell parameters decrease while the c parameter increases on the addition of Co2+. There are certain indications of structural instability for high Co-content compositions. (c) 2006 Elsevier B.V All rights reserved.