An electrochemical system at the ionic liquid (IL) | oil (O) interface has been constructed and utilized as electrochemical reaction field for reductive deposition of metal nanostructures. The interface between 1-(3-hydroxypropyl)-3-methylimidazolium chloride (C3OHmimCl), a hydrophilic IL, and 1,6-dichlorohexane (containing an organic electrolyte) exhibits a polarized potential window of 150 mV, which is limited by the ion transfer (IT) of the IL cation and anion at the positive and negative edges, respectively. The polarizable IL | O interface has allowed to record voltammograms for the electron transfer (ET) and IT processes across the IL | O interface that are involved in the reductive deposition of gold at the IL | O interface. The ET between AuCl4− in the IL phase and decamethylferrocene in the O phase proceeds without applying external voltage by coupling with the IT of AuCl4−, spontaneously forming Au nanostructures at the IL | O interface.