The chemical constituents of the protective surface film formed on graphite negative electrode in ethylene carbonate (EC)-based electrolyte solutions were analyzed by pyrolysis/gas chromatography/mass spectroscopy (Pyro/GC/MS). Oligomers that consist of oxyethylene units, such as ethylene glycol, di(ethylene glycol), and tri(ethylene glycol) methyl ester, were detected. These oligomers were formed by reductive decomposition of EC, and their presence suggested that the surface film contained polymer-like substances that have repeated oxyethylene units.