LiNi1/3Mn1/3Co1/3−z(MnMg)z/2O2 (0 ≤ z ≤ 1/3) samples were prepared from hydroxide precursors. The hydroxide precursors were heated with Li2CO3 at 900 °C for 3 h to prepare the oxides. The XRD results show single phase LiNi1/3Mn1/3Co1/3−z(MnMg)z/2O2 samples can be prepared for 0 ≤ z ≤ 1/3. Rietveld refinements of XRD data show that when Mn and Mg substitute for Co in NMC there is an increase in the amount of transition metal in the Li layer. Electrochemical studies of the LiNi1/3Mn1/3Co1/3−z(MnMg)z/2O2 (0≤ z ≤ 1/3) samples were used to measure the rate of capacity reduction with z, found to be about −250 (mAh/g)/(z = 1) charging to 4.3 V and −150 (mA h/g)/(z = 1) charging to 4.6 V. The impact of Mn and Mg cosubstitution on the thermal stability of charged LiNi1/3Mn1/3Co1/3−z(MnMg)z/2O2 (0 ≤ z ≤ 1/3) samples in electrolyte was studied using accelerating rate calorimetry. The thermal stability of LiNi1/3Mn1/3Co1/3−z(MnMg)z/2O2 is independent of z for the samples studied.