The structure of the coprecipitated precursor to the positive electrode material, Li[Ni1/3Mn1/3Co1/3]O-2 (NMC), is typically synthesized via a coprecipitation reaction and assumed to be Ni1/3Mn1/3Co1/3(OH)(2) or Ni1/3Mn1/3Co1/3OOH. The coprecipitation reaction and its products are not thoroughly understood. Here, pure phase Ni1/3Mn1/3Co1/3(OH)(2) is synthesized in the presence of aqueous ammonia, yielding dense and spherical particles. Ni1/3Mn1/3Co1/3(OH)(2) was found to readily oxidize in air, especially during heating. The obtained product was analyzed with powder X-ray diffraction and thermogravimetric analysis. The results indicate that Ni1/3Mn1/3Co1/3(OH)(2) undergoes oxidation to the oxyhydroxide phase, Ni1/3Mn1/3Co1/3OOH. Heating also resulted in an increase in tap density of the oxyhydroxide; the final tap density of the material was found to be 2.0 g cm(-3). (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3079366] All rights reserved. Intensity