In situ spectroscopic ellipsometry was employed to study the initial stage of solid electrolyte interphase (SEI) layer formation on thin-film LiMn2O4 electrodes. The SEI layer formed immediately upon exposure of the electrode to ethylene carbonate/dimethyl carbonate (1:1 by vol) 1.0 M LiPF6 electrolyte. The SEI layer thickness then increased in proportion to a logarithmic function of elapsed time. In comparison, the SEI layer thickness on a cycled electrode increased in proportion to a linear function of the number of cycles. (c) 2005 The Electrochemical Society.