It is found that the solid electrolyte interphase (SEI) film on LiCoO2 is not drifted from the anode side. It can be formed on the surface of nanosized LiCoO2 particles after storage in dimethyl carbonate (DMC) solvent. This is proved by transmission electron microscopy, Fourier transform infrared spectroscopy, and selected-area electron diffraction. The thickness of the SEI film is 2-5 nm and ROCO2Li, ROLi and Co3O4 are products from this spontaneous reaction of nanosized LiCoO2 and DMC. A reaction mechanism related to the reduction of DMC is proposed accordingly. It is also found that the SEI film becomes more uniform upon charging/discharging cycling. (c) 2006 The Electrochemical Society.