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In the present work, the extent and the role of oxygen release during the first charge of lithiumand manganese-rich Li1.17[Ni0.22Co0.12Mn0.66]0.83O2 (also referred to as HE-NCM) was investigated with on-line electrochemical mass spectrometry (OEMS). HE-NCM shows a unique voltage plateau at around 4.5 V in the first charge, which is often attributed to a decomposition reaction of the Li-rich component Li2MnO3. For this so-called “activation”, it has been hypothesized that the electrochemically inactive Li2MnO3 would convert into MnO2 while lattice oxide ions are oxidized and released as O2 (or even CO2) from the host structure. However, qualitative and quantitative examination of the O2 and CO2 evolution during the first charge shows that the onset of both reactions is above the 4.5 V voltage plateau and that the amount of released oxygen is an order of magnitude too low to be consistent with the commonly assumed Li2MnO3 activation. Instead, the amount of released oxygen can be correlated to a structural rearrangement of the active material which occurs at the end of the first charge. In this process, oxygen depletion from the HE-NCM host structure leads to the formation of a spinel-like phase. This phase transformation is restricted to the near-surface region of the HE-NCM particles due to the poor mobility of oxide ions within the bulk. From the evolved amount of O2 and CO2, the thickness of the spinel-like surface layer was estimated to be on the order of ≈2–3 nm, in excellent agreement with previously reported (S)TEM data. © The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. [DOI: 10.1149/2.1001702jes] All rights reserved.