Electrochemical lithium intercalation into graphite has been studied in ethylene carbonate (EC) based electrolytes using x-ray diffraction (XRD) and electrochemical quartz crystal microbalance techniques. The graphite electrode after cathodic polarization in EC mixed with dimethyl carbonate (DMC) showed a different XRD pattern from that polarized in EC mixed with propylene carbonate (PC). Changes in the resonance frequency of the graphite-coated quartz crystal proved that the cathodic intercalation of lithium into graphite is accompanied by the electrochemical decomposition of the electrolyte. The mass change per quantity of electricity in the potential range of 0.0 to 0.2 V vs. Li/Li+ was higher in EC+DMC than in EC+PC.