The electrochemical system presented here is the oxidation of ferrocene in the hemispherical droplet of polydimethylsiloxane (PDS) at the ring shape of the oil I water I electrode interface. The PDS phase included ferrocene without supporting electrolyte, whereas the aqueous phase included an electrolyte such as NaBF4 or NaClO4 without electroactive species. The voltammogram showed the steady-state anodic wave for the oxidation of ferrocene, independent of potential sweep rate. The limiting current was proportional to the radius of the droplet. These facts indicate that the electrode reaction should occur at the thin annular part on the electrode into which the water and the oil phase merge. The ring width of the three-phase boundary was evaluated from the limiting current on the basis of the microband model to yield 18 and 9 mum for BF4- and ClO4-, respectively, much larger than the molecular length or thickness of a diffuse double layer predicted geometrically from the intersection of two planes. The large values suggest participation in the fluctuation of the three-phase boundary. The fluctuation is ascribed to the emulsion formed at the oil\water interface owing to forced dissolution of ferricenium ion in the oil phase. The emulsion was observed with a microscope and was detected chemically. (C) 2004 Elsevier B.V. All rights reserved.