The relative stability of the lithiated-spinel structure, Li 2 [Co 2 ]O 4 , at 400 o C to the layered LiCoO 2 structure has been investigated. ''Low-temperature'' LT-LiCoO 2 samples were synthesized at 400 o C by the solid-state reaction of Li 2 CO 3 with CoCO 3 (or Co 3 O 4 ) for various times between 10 min and 232 days. Least-squares refinements of X-ray powder diffraction patterns were used to determine the fractions of lithiated-spinel Li 2 [Co 2 ]O 4 and layered LiCoO 2 in the samples. X-ray powder diffraction and transmission electron microscope data show that Li 2 [Co 2 ]O 4 nucleates from an intermediate Li x Co 1 - x [Co 2 ]O 4 spinel product before transforming very slowly to layered LiCoO 2 . The experimental data confirm the theoretical prediction that layered LiCoO 2 is thermodynamically more stable than the lithiated-spinel structure at 400 o C and support the arguments that a non-ideal cation distribution in Li 2 [Co 2 ]O 4 , non-stoichiometry and kinetic factors restrict the transformation of the lithiated-spinel structure to layered LiCoO 2 at this temperature.