The electrochemical intercalation of Li into graphite has been studied in Li/polymer electrolyte/graphite cells using an in situ x-ray diffraction (XRD) technique. In cells containing an electrolyte of PAN(polyacrylonitrile)-EC(ethylene carbonate)-LiPF6, a minor irreversible reduction of the electrolyte is observed only during the first discharge. In these cells, Li is reversibly intercalated into graphite to form Li1.0C6, principally at potentials between 0.2 and 0.0 V vs. Li+/Li. No evidence for the cointercalation of EC was obtained. In cells containing PAN-EC/PC(propylene carbonate)-LiPF6-based electrolyte, a massive reduction of electrolyte occurs during the first discharge at similar to 0.8 V vs. Li+/Li, which precludes Li intercalation into graphite. In situ XRD data are consistent with the absence of the intercalation of PC or Li+(PC)(2) solvates into the graphite lattice, either prior to or during the solvent reduction process. The latter appears to be a surface-catalyzed process, the extent of which is determined by a combination of thermodynamic and kinetic factors including the reduction potential of the electrolyte, and the passivating films which form on the graphite surface as a result of electrolyte reduction.