Vibrational sum-frequency generation spectroscopy (SFG) is used for in situ probing of molecular vibrations at interfaces associated with solid-electrolyte interphases (SEI) in systems relevant to lithium-ion batteries. SFG is interface-selective and can suppress nonresonant signals from metal electrodes. Two interfaces were observed: the electrode-SEI interface and the electrolyte-SEI interface. The SEI was formed on Au or Cu by potential cycling from 2.0 V-0.2 V (vs. Li/Li+) in ethylene carbonate (EC) and LiClO4 in tetrahydrofuran (THF). Li deposition occurs on Au but not on Cu. SFG of the electrolyte-SEI interface with Cu shows EC transitions whose intensities oscillate during potential cycling. The smaller oscillations are attributed to EC potential-dependent reorientation; the larger to an optical interference effect associated with the SEI layer thickness. The larger EC interference oscillations seen on Cu are absent on Au because the SEI on Au is thicker and more opaque. Lithium ethylene dicarbonate (LiEDC) and possibly ethylene oxide are observed at the electrode-SEI interface. This interfacial structure varies little after the first cycle of SEI formation. THF is also observed at the electrode interface with a degree of mobility that increases during the first few four potential cycles and then levels off. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.022203jes] All rights reserved.