The formation of passive film at the interface between the mesocarbon microbeads (MCMB) electrode and the organic electrolyte in a lithium-ion battery during the initial period of intercalation was investigated by a.c.-impedance spectroscopy. An equivalent-circuit model consisting of five parallel RC-circuits in series combination was adopted for the curve-fitting analysis of the obtained impedance spectra. The results indicated that both the total interfacial resistance and the passive film thickness increased with decreasing intercalation potential in the ethylene carbonate (EC) or dimethyl carbonate (DMC) single-solvent system, whereas an opposite trend was observed in the system containing diethyl carbonate (DEC) only. In addition, the total interfacial resistance was clearly affected by the porous structure of the passive film in a single-solvent system. In binary solvent systems such as EC/DEC and EC/DMC, on the other hand, the effect of the porous structure on the total interfacial resistance was negligible. The total interfacial resistance and the passive film thickness were also smaller in these systems than those in single-solvent systems. Finally, the variation of the total interfacial resistance and of the passive film thickness in the EC/DEC (or EC/DMC) system were also found to be similar to those in the parent DEC (or DMC) system during intercalation.