The solid electrolyte interphase (SEI) formed on a lithium electrode in an N-methyl-N-alkylpyrrolidinium bis(trifluoromethane-sulfonyl)amide [p(1,x)(Tf)(2)N] room-temperature ionic liquid electrolyte was characterized using X-ray photoelectron spectroscopy, diffuse reflectance Fourier transform infrared spectroscopy, Raman spectroscopy, and electrochemical impedance spectroscopy (EIS). The SEI was found to be composed mainly of reduction products of the (Tf)(2)N-anion. A pronounced difference in composition was observed between the SEI formed on the lithium surface and that formed in situ during lithium deposition on a copper substrate. In the case of the lithium surface, native surface species (e. g., Li2O, Li2CO3) persisted in the SEI and dominated the SEI composition. The surface film formed on lithium-deposited-on-copper did not contain species associated with the lithium native film. Instead, in addition to the anion reduction products, significant quantities of species associated with the cation were observed. EIS indicated varied lithium conduction pathways through the film and that the pathways were in series, suggesting a layered structure. Calculated activation energies, resistivity, and thickness values were comparable to literature values for the SEI formed in conventional liquid electrolytes. (c) 2006 The Electrochemical Society.