Conventional electrochemical techniques and a fast-scan, self-compensating spectroscopic ellipsometer were used to characterize Li/polymer electrolyte interfaces. An electrochemical–ellipsometric test cell employed a grid counter-electrode to allow illumination of the Li/polymer interface, along with a Li-ring reference electrode, which was co-planar with the Li working electrode. Open-circuit cell potentials, current responses to small potential steps, AC impedance data and in situ ellipsometric spectra were recorded after cell assembly and during cell cycle tests, and the optical properties of bare Li metal and candidate reaction products were measured. The ellipsometric spectra of Li/PEO–LiN(CF3SO2)2 interfaces were interpreted and were consistent with a compact interfacial structure. The as-formed (before current passage) Li/PEO–LiN(CF3SO2)2 interfacial layer was ∼12-nm thick and contained mostly Li and Li2O, with >60 vol.% Li. After a single-cell discharge of 0.4 mA h/cm2, the interfacial layer became thicker (37 nm) and accumulated >60 vol.% Li2O.