The degradation mechanism due to the decomposition of organic electrolytes during charge and discharge cycling of Li/MoS2 cells has been investigated. The gas products of the electrolyte decomposition during long cycling were measured by gas chromatography (GC), and the changes of electrolyte composition was measured by gas chromatography/mass spectrometry (GC/MS). The gases were produced through two types of electrolyte decomposition: the electrochemical reaction, which was dependent on the charge and discharge voltage, and the chemical reaction, which was independent of the discharge voltage. This decomposition of electrolytes occurred under a discharge voltage lower than 1.4 V. The operating conditions with a lower discharge rate and a deeper depth of discharge (D.O.D.) accelerated the electrochemical decomposition. The rate of gas production by chemical decomposition, which is independent of the discharge condition, was estimated to be about 0.1 ml/h.