Chloroethylene carbonate (ClEC) is decomposed to CO2 at graphite electrodes. We assume that the CO2 participates in the formation of an effective solid electrolyte interphase (SM) on the electrode. Two in-situ techniques, subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) and differential electrochemical mass spectrometry (DEMS), were applied in order to detect CO2 formation and possible secondary reactions. The applied analytical methods provided conforming information about the onset of CO2 formation (2.2-2.1 V vs. Li/Li+).