Surface tension and impedance measurements were used to study the electrochemical properties of the interface between 0.01 hi LiCl in water and 0.01 M tetrabutylammonium tetraphenylborate in 1,2-dichloroethane in the absence and the presence of the Cs+ or tetraethylammonium ion. While the presence of these ions has no effect on the surface tension, the interfacial capacity is considerably enhanced and exhibits a peculiar correlation with the faradaic admittance of the Cs+ or tetraethylammonium ion transfer reaction. Changes in the capacity appear to be artefacts arising from the inadequate description of the electrical behaviour of the interface in terms of the Randles-type equivalent circuit employed in the analysis of experimental impedance data. A more convenient description is provided by our model, which links the measured impedance to the mechano-electric oscillations in the interfacial region during the a.c. impedance measurements.