The interface of a Li4Ti5O12/electrolyte in 0.0-5.0 V (vs Li/Li+) is thoroughly studied by cyclic voltammetry, high-resolution transmission electron microscopy, and Fourier transform infrared spectroscopy. Solid electrolyte interphase (SEI) film formed on the particle surface is observed and analyzed in low (<1.0 V) and high (>3.5 V) potential ranges. The passivating layer deposited during the different electrochemical processes has different composition. Inorganic fluorides and copolymers are the final products for electrolyte oxidation in high potential range. Organic lithium alkylcarbonates are the primary components of SEI film formed during the reduction process. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.2992706] All rights reserved.