The chemistry of the solid electrolyte interface (SEI) layer formed on graphite anodes cycled in propylene carbonate (PC)-lithium bis(oxolato)borate (LiBOB) electrolyte was studied by ex situ Fourier transform infrared spectroscopy analysis in the attenuated total reflection mode. The vibrational spectra clearly show that electrochemical reduction of the BOB anion is a part of the SEI layer formation chemistry. The reduction reaction changes the oxygen coordination around the B atom from tetrahedral BO4 to trigonal BO3, e.g., esters of boric acid, and changes the symmetry of the O-C-O bonding closer to that in esters of oxalic acid. (C) 2004 The Electrochemical Society.