Rate constants for the reduction of tris(2,2-bipyridyl)-dichlororuthenium (III) (Ru(bipy)(3)(3+)) in water by 7,7,8,8-tetracyanoquinodimethane radical anion (TCNQ(.-)) in 1,2-dichloroethane (DCE) have been measured as a function of the Galvani potential across an ideally non-polarizable interface using scanning electrochemical microscopy (SECM). The Galvani potential was varied by changing the concentration of tetrabutylammonium chloride in the aqueous phase, with a fixed concentration of the organic electrolyte, tetrabutylammonium tetrakis(penta-fluoro)phenylborate, employed in the DCE phase. By selecting appropriate concentration ratios of the reactants in the two phases, fast electron transfer (ET) rate constants were measured which were found to increase with increasing driving force. The experimentally obtained ET rate constants have been compared to the corresponding theoretical values calculated from the sharp boundary and thick layer models of Marcus. The results are consistent with the sharp boundary model, providing further evidence for the applicability of this model for the description of simple ET processes across liquid/liquid interfaces. (c) 2007 Elsevier B.V. All rights reserved.