Online electrochemical mass spectrometry (OEMS) was employed to investigate the behavior of ethylene carbonate (EC) and fluoroethylene carbonate (FEC) co-solvents in high voltage lithium-ion batteries based on high energy lithium nickel cobalt manganese oxide (HE-NCM) and SFG6 graphite (SFG6) electrodes. Gas evolution from HE-NCM and SFG6 electrodes was studied during two charge/discharge cycles in half-cell and full-cell configuration using electrolytes composed of 1 M LiPF6 in either 3:7 (w/w) EC:DEC or 3: 7 (w/w) FEC: DEC. The results suggest that some CO2 formation is caused by reactive oxygen species originating from Li2MnO3 domain activation. The electrolyte composition has no effect on O-2 evolution during Li2MnO3 domain activation and oxidative CO2 evolution in HE-NCM/Li half-cells. In contrast, decomposition of PF6- is significantly enhanced in electrolyte containing FEC and found to be strongly facilitated by the SuperC65 conductive carbon additive employed in the HE-NCM electrodes. EC and FEC clearly follow different reductive decomposition pathways. This leads to major differences in solid electrolyte interphase (SEI) formation and affects follow-up reactions involving CO2 and possibly other processes relevant for the stability and endurance of high voltage lithium-ion batteries. (C) The Author(s) 2016. Published by ECS. All rights reserved.