The electrolyte additives tris(trimethylsilyl)phosphite (TMPSi) and tris(trimethylsilyl)phosphate (TMSPa) have shown their potential to improve lifetime and decrease impedance build-up in lithium ion batteries (LIBs) in several studies. Online electrochemical mass spectrometry (OEMS) was applied to study the effect of TMSPi and TMSPa on a gas releasing parasitic reaction during cycling of HE-NCM/graphite cells in different electrolytes to elucidate their working mechanisms, for which so far contradictive claims can be found in the literature. The results underline the chemical instability of the two additives in solution and show that they have, with one exception, little or no effect on the release of O₂ and CO₂ which would be expected in the case of a passivating surface layer suppressing electrolyte decomposition. The decreased POF₃ evolution and the formation of Me₃SiF indicates that the main role of both additives is that of a chemical scavenger, e.g., for HF, which leads to decreased acidification of the electrolyte and thus a longer lifetime of LIBs containing these electrolytes.