Electrochemical lithium intercalation into graphite and related carbonaceous materials has been investigated in ethylene carbonate (EC)-based organic electrolytes using electrochemical quartz crystal microbalance (EQCM). During constant-current charging (cathodic polarization), changes in the resonance frequency indicated the mass of the electrode increasing generally with an increase in charge passed. The mass change of the graphite per quantity of electricity (Delta m Delta Q(-1)) depended on the electrolyte composition in the potential range of 0.0-0.2 V versus Li/Li+ as well as in the more positive potential range. Variations in Delta m Delta Q(-1) with electrode potential revealed that the relative rate of electrolyte decomposition yielding precipitation on the graphite surface changes with electrode potential. For the artificial graphite, the mass change in EC + DMC (dimethyl carbonate) containing LiClO4, was larger than that in EC + PC (propylene carbonate) containing LiClO4, which suggested that the process in EC + DMC is somewhat different from that in EC + PC. These results are discussed in connection with those obtained from conventional electrochemical and X-ray diffraction measurements.