Coupled electron and ion transfer was studied at a three-phase arrangement where decamethylferrocene was dissolved in a droplet of nitrobenzene that was sandwiched between an optically transparent ITO electrode and a glass plate. The sandwiched droplet was surrounded by an aqueous electrolyte solution in which the auxiliary and the reference electrodes were positioned. The oxidation of decamethylferrocene was followed by in situ spectroscopy with the help of a spectrometer interfaced to an optical microscope. The fast progression of the reaction starting at the three-phase junction and advancing towards the middle of the droplet is explained on the basis of a simplified model that assumes a progressive activation of the working electrode I nitrobenzene interface due to the ingress of transferred anions and decrease of ohmic resistance inside the nitrobenzene phase. (C) 2003 Elsevier B.V. All rights reserved.