We report the first direct measurement of the extent of the spontaneous non-electrochemically driven reaction between a lithium ion battery electrode surface (Si) and a liquid electrolyte (1.2 M LiPF₆–3 : 7 wt% ethylene carbonate : dimethyl carbonate). This layer is estimated to be 35 Å thick with a SLD of ∼ 4 × 10⁻⁶ Å⁻² and likely originates from the consumption of the silicon surface.