The formation of a solid electrolyte interphase (SEI) before and during lithium intercalation was studied on graphite electrodes in ethylene carbonate based electrolytes. We demonstrated by using in situ mass spectrometry that during the first charge of the graphite electrode ethylene gas is evolved in a potential window that corresponds to the formation of a SEI. Moreover, development of hydrogen gas was detected even in dry electrolytes containing < 10 ppm H2O. No CO2 is developed however, as confirmed by two in situ methods, mass spectrometry and infrared spectroscopy. We conclude that the formation of the SEI is a complex process which depends among other things on the amount of trace water present in the cell. In addition, in situ Raman mapping experiments revealed that lithium intercalation into graphite does not proceed homogeneously.