Two Li[Ni0.2Co0.6Mn0.2]O-2 samples were synthesized by firing Ni0.2Co0.6Mn0.2(OH)(2) coprecipitate mixed with either LiOH or Li2CO3. Another two Li[Ni0.2Co0.6Mn0.2]O-2 samples were made from Ni0.416Co0.168Mn0.416(OH)(2) coprecipitate mixed with Co(OH)(2) and LiOH or Li2CO3. All samples were single phase by X-ray diffraction. The structure and electrochemical properties of the synthesized Li[Ni0.2Co0.6Mn0.2]O-2 samples were compared and the reactivity of the four charged Li-x[Ni0.2Co0.6Mn0.2]O-2 (4.2 V) samples with electrolyte was examined using accelerating rate calorimetry. All four charged Li-x[Ni0.2Co0.6Mn0.2]O-2 (4.2 V) samples show less reactivity than LixCoO2 (4.2 V) in ethylene carbonate/diethyl carbonate solvent or in LiPF6-based electrolyte. However, Li[Ni0.2Co0.6Mn0.2]O-2 synthesized from Ni0.2Co0.6Mn0.2(OH)(2) mixed with LiOH or Li2CO3 shows higher thermal stability than Li[Ni0.2Co0.6Mn0.2]O-2 samples made from Ni0.416Co0.168Mn0.416(OH)(2) coprecipitate mixed with Co(OH)(2) and LiOH or Li2CO3, even though the particle size of the latter materials is larger. The reasons for this surprising result are explained. Because the safety of Li[NixCo1-2xMnx]O-2 materials is strongly dependent on x, near x = 0, the safest materials are those with the most homogeneously mixed cations. (c) 2005 The Electrochemical Society.