Experimental studies of electron-transfer (ET) reactions at the interface between two immiscible electrolyte solutions (ITIES) were carried out with Fe(CN)(6)(4-) as a reductant in water and 7,7,8,8-tetracyanoquinodimethane as an electron acceptor in 1,2-dichloroethane (DCE). The kinetics of this process have been determined using both scanning electrochemical microscopy and microelectrochemical measurements at expanding droplets. ClO4- was employed in each phase to control the interfacial potential drop. The ET rate constants were found to depend on the interfacial potential drop, with an apparent ET coefficient in the range 0.30-0.41 with a moderate aqueous ionic strength 0.1-0.3 M and an electrolyte concentration in the DCE phase of 0.1 M. When the aqueous ionic strength was increased significantly to the levels employed in studies of externally polarized ITIES, the ET rate constant decreased appreciably and was less dependent on the interfacial potential drop. These latter studies yielded results comparable to those obtained at externally polarized ITIES.