Li1+x (Ni1/3Mn1/3Co1/3)(1-x) O-2 (NMC) oxides are among the most promising positive electrode materials for future lithium-ion batteries. A voltage "plateau" was observed on the first galvanostatic charging curve of NMC in the extended voltage region positive to 4.5 V vs. Li/Li+ for compounds with x > 0 (overlithiated compounds). Differences were observed in the cycling stability of the overlithiated and stoichiometric (x = 0) NMC oxides in this potential region. A differential plot of the charge vs. potential profile in the first cycle revealed that, for the overlithiated compounds, a large irreversible oxidative peak arises positive to 4.5 V vs. Li/Li+, while in the same potential region only a small peak due to the electrolyte oxidation is detected for the stoichiometric material. Differential Electrochemical Mass Spectrometry (DEMS) was used to investigate the high voltage region for both compounds and experimental evidence for oxygen evolution was provided for the overlithiated compounds at potentials positive to 4.5 V vs. Li/Li+. No oxygen evolution was detected for the stoichiometric compound.