A criterion of the electrochemical polarizability of liquid-liquid interfaces, that is, the controllability of the phase-boundary potential has been proposed for the interface between a hydrophobic room-temperature molten salt (ionic liquid) and an aqueous electrolyte solution. When the partition of each ionic species constituting the molten salt between the two-phases follows the Nernst equation, the electrochemical polarizability is directly correlated with the solubility product of the RTMS and also with the width of the polarized potential window, provided that electrolytes in the aqueous phase are so hydrophilic that their dissolution to the molten salt phase is negligible. Voltammograms at the interface between 1-octyl-3-methylimidazolium bis(perfluoroalkylsulfonyl)imide and water are well explained by this model. (c) 2006 Elsevier B.V. All rights reserved.