The electron transfer reaction across the 1,6-dichlorohexane (1,6-DCH)\water (W) interface where bis(pentamethylcyclopentadienyl)iron(II/III) ([Fe-II/III (C5Me5)(2)](0/+)) as the organic redox couple in 1,6-DCH is present in excess has been studied by cyclic voltammetry. The 1,6-DCH\W interface gave a potential window where the transfer of [Fe-III(C5Me5)2](+) ion as well as that of [Fe-II(C5Me5)2](0) across the interface can be neglected. When hexaammineruthenium(III) and cyano complexes of iron protoporphyrin IX and microperoxidase-11, which do not transfer across the 1,6-DCH\W interface in the potential range tested, are present in W as the aqueous reactant, well-defined voltammetric waves due to the heterogeneous electron transfer have been observed within the potential window. The voltammetric waves can be assigned to reversible one-electron transfer controlled by diffusion of the aqueous redox couple in W. The reversible half-wave potentials are consistent with those predicted from simple voltammetric experiments with a Pt electrode. (C) 2003 Elsevier B.V. All rights reserved.