The formation and stabilities of interfaces between poly[oxymethylene oligo-(oxyethylene)] (PEM)-salt polymer electrolytes and lithium metal or LiCx are investigated. The Li/PEM(25)LiClO(4) interface (where x in PEM(x)LiAn indicates the mole ratio of O/Li in the electrolyte) demonstrates behavior consistent with the formation of a solid electrolyte interface : with a stable interfacial resistance of 900 Omega cm(2) at 50 degrees C. UV-cross-linked films do not form a stable interface, R(int) rapidly increases to greater than 10(5) Omega cm(2). When cross-linked samples are treated with methyl iodide, hexamethyldisilazane/trimethylchlorosilane or upon addition of the plasticizer polyethylene glycol dimethyl ether, R(int) at 50 degrees C stabilizes at approximately 2000 Omega cm(2). The polymer electrolytes require a plasticizer to form a low impedance interface with LIC(x).