Single-phase O3-type LixCoO2 (0.5 <= x <= 1.0) samples were synthesized by chemically extracting lithium from LiCoO2 with an oxidizer, NO2BF4, in acetonitrile medium under an argon atmosphere. The enthalpy of formation of LixCoO2 at 298 K was determined by high-temperature oxide melt solution calorimetry. The enthalpy of formation from binary oxides plus oxygen for LixCoO2 at 298 K varies linearly with x, and LixCoO2 becomes energetically less stable with decreasing x. From the linear trend, the enthalpies of formation for O3-type CoO2 and monoclinic Li0.5CoO2 were estimated. Monoclinic Li0.5CoO2 is energetically more stable than the undistorted O3-type Li0.5CoO2. For CoO2, the energetic stability decreases in the order O1 > P3 > O3. (c) 2005 The Electrochemical Society. All rights reserved.