Li1+x(Ni1/3Mn1/3Co1/3)(1-x)O-2 (NMC) oxides are believed to be among the most promising positive electrode materials for future lithium-ion batteries. Differential electrochemical mass spectrometry (DEMS) experiments on the NMC oxides revealed oxygen evolution for overlithiated NMC compounds (x = 0.1) at potentials positive to 4.5 V vs Li/Li+. No oxygen evolution was detected for the stoichiometric NMC compound (x = 0). An analytical procedure for the quantitative analysis of the DEMS data was developed and applied to the oxygen evolution from overlithiated NMC compounds in acetonitrile (AcN) and ethylene carbonate/dimethylcarbonate-based electrolytes. The analysis of DEMS data acquired in the AcN-based solution was used to quantify the oxygen loss from the structure of the overlithiated compounds. A hypothesis on the origin of the considerably irreversible specific charge observed in the first cycle of the galvanostatic cycling experiments on the overlithiated compounds was formulated. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3205495] All rights reserved.