LiCo1-z[MnMg](z/2)O-2 (0 <= z <= 0.7) samples were prepared from Co1-z[MgMn](z/2)(OH)(2) precursors. The hydroxide precursors were heated with Li2CO3 at 900 degrees C for 3 h to prepare the oxides. Electrochemical studies of the LiCo1-z[MnMg](z/2)O-2 (0 <= z <= 0.3) samples were used to measure the rate of capacity reduction with Mn + Mg content, found to be about -130 (mAh/g)/(z = 1). The results of in situ X-ray diffraction and differential capacity measurements show that irreversible loss of oxygen, along with lithium extraction, from the compound with z = 0.3 occurs during the first charge between 4.4 and 4.6 V. The impact of Mn and Mg cosubstitution on the thermal stability of LiCo1-z[MnMg](z/2)O-2 samples was studied via accelerating rate calorimetry. There is little or no improvement in thermal stability caused by Mn and Mg cosubstitution. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3457425] All rights reserved.