Adsorption of the nonionic surfactant, Triton X-100, at the interface between two immiscible electrolyte solutions (ITIES) and its effect on the oxidation of decamethyl ferrocene in 1,2-dichloroethane by aqueous Ru(CN)(6)(3-) has been investigated using scanning electrochemical microscopy. As the concentration of Triton X-100 in the aqueous phase was increased from 0 to 2.5 x 10(-4) M, the rate constant for electron transfer (ET) across the ITIES decreased in a manner consistent with the surfactant blocking the area available for the reaction. The decrease in rate constant with increasing surfactant concentration was successfully analyzed in terms of Langmuirian adsorption of the surfactant, with an equilibrium constant of (2.72 +/- 0.06) x 10(4) M-1. The overall behavior observed indicates that the ET reaction occurs primarily at the portion of the ITIES free from surfactant. (C) 2000 Elsevier Science B.V. All rights reserved.