Electrochemical oxidation of electrolytes becomes a more serious problem in high voltage lithium cells. In this paper, the reaction mechanisms of propylene carbonate (PC) and ethylene carbonate (EC) are investigated by using in situ mass spectrometry and gas chromatography-mass spectrometry. When PC is oxidized, it decomposes into carbon dioxide and propylene oxide, which in turn react to form other by-products, such as propanal and 2-ethyl-4-methyl-1,3-dioxolane. Carbon dioxide is generated during EC oxidation.