A nitrobenzene droplet including ferrocene (Fc) was attached to a glassy carbon electrode in aqueous solution so that a three-phase boundary between the water, the oil and the electrode was formed. The question arises as to whether Fc is oxidized at the oil \ electrode interface or the oil \ electrode \ water interface. Hermes and Scholtz proposed the latter interface [Electrochem. Commun. 2 (2000) 845]. We constructed two cells with oil \ electrode and oil \ electrode \ water interfaces, and compared voltammograms for the oil droplet with and without supporting electrolyte. From the dependence of the oxidation current of Fc on potential sweep rate, concentration of Fc, the number of potential scans and the contact area between the oil droplet and the electrode, we found that the peak current in the droplet with supporting electrolyte could be approximated by the conventional voltammetric equation except for the coefficient. In contrast, the effect of the three-phase boundary, i.e. linearity of the current with the radius of the contact area, is conspicuous at high potential scan rates in the droplet without supporting electrolyte. (() 2002 Elsevier Science B.V. All rights reserved.