Wide polarized potential windows against interfacial ion transfer are available at the interface between a hydrophobic ionic liquid (IL), tetraheptylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([THpA(+)][TFPB-]) and an aqueous electrolyte solution (W) of MgSO4. The width of the potential window, 1.1 V at 60 degrees C, enables electrochemical studies of charge transfer at the IL/W interface for a variety of ions and redox couples.