Interfacial reactions in lithium-ion batteries often involve gaseous reaction products. Mechanistic investigation of material degradation processes requires a technique to identify and quantify these gases in battery cells. On-line electrochemical mass spectrometry (OEMS) is an operando gas analysis method that continuously samples the headspace of a custom battery cell. Real-time gas analysis by quantitative OEMS was used to create mechanistic understanding of battery degradation reactions, some of which will be highlight in this article.