Electrochemical impedance spectroscopy (EIS) and in-situ intrinsic resistance measurements were applied to sandwiched disk graphite electrodes to investigate the mechanism of formation of irreversible capacity during initial galvanostatic or potentiostatic lithium intercalation into graphite in four different electrolyte formulations. The stability of the solid I electrolyte interphase (SEI) film formed during pre-cycling eight times between 1.5 and 0.2 V versus Li \ Li+ in 1.0 M LiPF6 + ethylene carbonate (EC) based electrolytes was studied using EIS and in-situ intrinsic resistance measurements during subsequent cycling in the same electrolyte between 0.4 and 0.0 V, and in I M LiPF6-propylene carbonate (PC) between 0.4 and 0.0 V. The purpose of the first set of results was to investigate the effect of the volume change resulting from Stage transformation during Li intercalation - deintercalation on the stability of the previously formed SEI film. The second measurements were taken to study the reaction of the corresponding film in PC electrolyte. Results show that the two potential plateaus at around 0.9 and 0.6 V in the first galvanostic charge largely correspond to solvated Li intercalation into graphite with some contribution from the formation of the SEI film. In EC-containing mixtures, this solvent has little influence on co-intercalation at 0.9 V, but can effectively inhibit the process at 0.6 V. Potentiostatic charging at 0.2 or 0.7 V accelerates co-intercalation. Although the SEI film formed during cycling in EC-containing electrolytes can inhibit PC co-intercalation during subsequent cycles in the presence of this solvent, a new layer of different SEI film may also form.