Since the potential for alloying lithium with silicon is outside the window of stability of common commercial electrolytes, silicon surfaces form an amorphous solid electrolyte interphase (SEI) under applied potential, which hampers silicon's performance as a lithium-ion battery anode. We have investigated the composition, distribution, and ambient stability of the SEI formed on undoped silicon (001) wafers configured as model electrodes in three different electrochemical conditions using a reduced oxidation interface for transporting air-sensitive samples from a glovebox to an ultra-high-vacuum chamber for X-ray photoelectron spectroscopy (XPS) analysis. Variable potential cycling and step experiments included linear sweep voltammetry (LSV), cyclic voltammetry (CV), and chronoamperometry (CA). CV and LSV experiments on silicon electrodes scanned from open-circuit voltage to lithiation (3-0.01 V vs Li/Li +) showed a suppression of carbonate-containing species relative to CA experiments (potential step for 300 s at 0.01 V vs Li/Li +) in anoxic XPS measurements. When silicon electrodes were exposed to ambient air, SEI layers reacted through both fluorination and combustion processes to produce different SEI product distributions than those prepared under anoxic conditions.