Electron transfer reactions of the form, ML63+(w) + R-n-FER(o) --> ML62+(w) + RnFER+(o), where ML63+,2+(w) are hydrophilic aquo or amine transition metal based redox couples and RnFER+,0(o) are hydrophobic alkyl-ferrocence redox couples, can occur at the interface between aqueous (w) and immiscible organic (o) phases. The hydroxymethylferrocene(+,0) couple was used as an internal standard in order to compare the formal reduction potentials for a variety of aqueous couples with reduction potentials for the ferrocene couples in organic solvents with dielectric constants ranging from 9 to 35. The ability of various electrolytes to provide adequate conductivity in aqueous/organic phases, without inducing partitioning of alkylferricenium cations into the aqueous phase, was examined. The single electrolyte tetraethylammonium tetrafluoroborate and the electrolyte consisting of potential determining ions tetrapropylammonium bromide/tetrapropylammonium tetraphenylborate were found to be generally suitable in these respects. Decamethylferrocenium ion was found to partition from organic to aqueous phases under certain conditions. In contrast, 1,1＇,3,3＇-tetrakis(2-methyl-2-hexyl)-ferrocenium remained in the organic phases. The rate of partitioning of alkylferricenium ions from benzyl cyanide thin films immobilized on carbon electrode surfaces was found to depend on the hydrophobicity of the cation and on the electrolyte ions. Estimates of the interfacial potential difference, Delta(o)(w)phi, induced by the two favored electrolytes for several aqueous/organic solvent interfaces were determined. These values of Delta(o)(w)phi combined with relative values of formal reduction potentials for aqueous and organic soluble redox couples can be used to estimate the driving force for a wide variety of electron transfer reactions at liquid/liquid interfaces. When the tetrapropylammonium ion, a potential determining ion, was used as the electrolyte, the value of Delta(o)(w)phi that was established conformed to the Nernst equation. The single electrolyte tetraethylammonium tetrafluoroborate established a value of Delta(o)(w)phi that was independent of salt concentration below 0.1 M.