Accelerating rate calorimetry (ARC) has been used to compare the thermal stability of three Li0.5CoO2 materials with different particle sizes (diameters of approximately 0.8, 2, and 5μm, respectively) when heated in 1.0M LiPF6 EC/DEC or 0.8M LiBoB EC/DEC electrolytes. The thermal stability of Li0.5CoO2 was found to be related to its particle size. The larger the particle size, the higher the onset temperature of self-heating as measured by ARC. In the presence of sufficient reducing agent, EC/DEC solvent, Li0.5CoO2 can be reduced to Co3O4, CoO, eventually even to Co metal. The heats of reaction for each of these steps are 550, 270 and 540J/g, respectively, measured per gram of Li0.5CoO2. The addition of LiPF6 salt significantly decreases the reactivity of Li0.5CoO2 compared to pure EC/DEC solvent. The reactivity of Li0.5CoO2 is stronger in 0.8M LiBoB EC/DEC than in LiPF6 EC/DEC.