Applications of voltages of more than 0.9 V vs. Ag \ AgOx to oil-in-water emulsions caused coalescence of ferrocene-included oil droplets on the electrode surface. The oxidation of ferrocene was responsible for the coalescence in that it made the oil droplets hydrophilic to reduce the surface tension. The electrochemical coalescence was observed with a video-microscope, and the video was transformed into a time-sequential variable representing time-variations of patterns. The contact angle of one oil droplet on the electrode was evaluated in the aqueous solution for various electrode potentials. The angle was more than 90degrees in the reduced state of ferrocene whereas it was less than 90degrees in the oxidized state. Two droplets on the electrode were used for the simplest model of the coalescence, and their geometrical variation with the potential was observed through a video. The coalescence occurred through the following processes: an increase in ionic concentration by the oxidation of ferrocene to ferricenium ion, a decrease in the contact angle by reduction of the hydrophobicity, a merging of the front edge of neighboring droplets on the electrode, an abrupt increase in the force at the merging point of two droplets normal to the electrode, and a combination of the two into a uni-phase. The emulsions were synthesized by mixing the ferrocene-included nitrobenzene in the sodium dodecylsulfonate aqueous solution. (C) 2003 Elsevier Science B.V. All rights reserved.