Although ether-based electrolytes such as tetraethylene glycol dimethyl ether (TEGDME) have been well recognized and widely employed for lithium–oxygen (Li–O₂) cells, researchers do not have a strong confidence in this electrolyte material, because there have been many contradictory results reported. The principal objective of this paper is to clarify whether TEGDME is truly stable and suitable for Li–O₂ battery operation. To accomplish the objective, oxygen efficiency and by-product gas evolution during five discharge/charge cycles were determined in a real-time and quantitative manner by in situ differential electrochemical mass spectrometry (DEMS). The amide-based electrolyte dimethylacetamide (DMA), which has recently been considered a promising electrolyte for Li–O₂ cells, was also investigated and compared. The quantitative DEMS data during five cycles clearly show that DMA is more stable and exhibits better performance than TEGDME, suggesting that DMA is a more favorable electrolyte for Li–O₂ battery applications. DMA exhibits 19% greater oxygen efficiency at charge, 5.1% lower CO₂ evolution, and 5% higher energy efficiency than TEGDME during the first cycle. As the discharge/charge operation process continues, the performance gap between the two electrolytes becomes wider. In particular, the gap in the oxygen efficiency at charge grows to 32% at the fifth cycle. Linear sweep voltammetry (LSV)-DEMS analysis with Li₂O₂-deposited Li–O₂ cells demonstrates that the evolutions of O₂ and CO₂ largely overlap, indicating that the oxidation of Li₂O₂ is inevitably accompanied by a parasitic reaction during the charge process with the TEGDME electrolyte.