The chemical reaction of diethyl carbonate (DEC) with lithiated graphite (LiC6) was studied in vacuum with X-ray photoelectron spectroscopy (XPS) using the temperature-programmed reaction methodology. DEC molecules were condensed onto the sample surface at 120 K, and XPS spectra were collected while warming the sample in increments of 30 to 620 K. For purposes of comparison, surfaces of pure metallic Li, lithiated graphite (LiC6), and bare graphite were studied using identical procedures. The reaction path of DEC on LiC6 surface was very similar to that on pure Li except for small shifts in the temperature where the same reaction occurred. In the range of 180-270 K, changes in the charging of the surface layer as the DEC evaporates/reacts on the surface make it difficult to determine the reaction products. In the 270-390 K region, the C/O stoichiometry of the surface layer decreases from 1.67 to ca. 1.0, and reaction products on the surface are most probably lithium methoxide (CH3O-) and lithium oxalate (C2O42-). Ethylene and/or CO were the only clearly identifiable gas-phase reaction products. Li2O begins to form at ca. 330-360 K from the decomposition of oxalate. Methoxide is thermally stable up to about 450 K, near the melting point of lithium. The final reaction at 390-620 K is decomposition of all organic carbon-oxygen species to form Li2O and CO. Our experiments thus indicate that the exotherm observed at ca. 120-140degreesC in the calorimetry of Li-ion negative electrode materials is most probably the decomposition of the solid electrolyte interface layer into Li2O and CO, and not into Li2CO3.