LiCoO2, LiNiO2 and their solid solution, LiNi1-xCoxO2 are important cathode materials for lithium ion batteries. Samples in this system were synthesized by solid state reaction of CO3O4, NiO and Li2CO3 or LiOH.H2O. Their lattice parameters were determined by Rietveld refinement. High temperature drop solution calorimetry in molten 3Na(2)O.4MoO(3) and 2PbO.B2O3 solvents at 974 K was performed to determine the enthalpy of formation from the constituent oxides plus oxygen and the enthalpy of mixing in the solid solution series. There are approximately linear correlations between the lattice parameters, the enthalpy of formation from oxides (Li2O, NiO and COO) plus 0, and the Co content in the compounds. The solid solution of LiCoO2 and LiNiO2, is almost ideal, showing a small positive enthalpy of mixing. The enthalpy of formation of LiCoO2, from oxides (Li2O, NiO and COO) and oxygen at 298 K is - 142.5 +/- 1.7 kJ/mol (from sodium molybdate calorimetty) or -140.2 +/- 2.3 kJ/mol (from lead borate calorimetry). That of LiNiO2 is -56.2 +/- 1.5 kJ/mol (from sodium molybdate calorimetty) or -53.4 +/- 1.7 kJ/mol (from lead borate calorimetry). The cobalt compound is thus significantly more stable than its nickel analogue. The phase assemblage LiCoO2, Li2O and CoO is seen at a lower oxygen pressure at constant temperature than the assemblage Co3O4/CoO, reflecting the stabilization of Co(III) in the ternary Li-Co-O system. (C) 2003 Elsevier B.V. All rights reserved.