Our electrochemical system is a hemispherical droplet of nitrobenzene including ferrocene, which is mounted on a glassy carbon electrode in a sodium sulfate aqueous solution. It is composed of a hemispherical oil \ water interface, a disk-shaped oil \ electrode interface, a water \ electrode interface and a ring oil \ water \ electrode interface. The voltammetric current of the droplet showed two anodic waves for the oxidation of ferrocene. The current at 0.5 V was proportional to the radii of the droplet and concentration of ferrocene, and was independent of the potential sweep rate as well as the concentration of sulfate. These facts indicate that the electrode reaction should occur at the thin ring on the electrode into which the water and the oil phase merge. A model of a microband electrode was suggested and was applied to the limiting current to evaluate the width of the three-phase interface. The evaluated width, 0.23 mum, is much larger than the molecular size. (C) 2003 Elsevier Science B.V. All rights reserved.