The electron transfer (ET) reaction between bis(cyclopentadienyl)iron(II) ([Fe-II(C5H5)(2)]) in 1,2-dichloroethane (1,2-DCE) and in nitrobenzene (NB) and hexacyanoferrate redox couple ([Fe-II/III(CN)(6)](4-/3-)) in water (W) at the interface has been studied by use of normal-pulse voltammetry. The voltammetric results indicate that the ET reaction between [Fe-II(C5H5)(2)] in O (O = 1,2-DCE or NB) and [Fe-II/III(CN)(6)](4-/3-) in W takes place not by way of the heterogeneous ET at O vertical bar W interface but by the mechanism in which a homogeneous ET reaction between [Fe-II(C5H5)(2)] (partially distributed from O) and [Fe-III(CN)(6)](3-) takes place in W phase and the resultant [Fe-III(C5H5)(2)](+) ion is responsible for the current passage across the interface. From the limiting current, the forward rate constants of the homogeneous ET reaction: [Fe(C5H5)(2)](+) [Fe(CN)(6)](3-) = [Fe(C2H5)(2)](+) + [Fe(CN)(6)](4-) in W phase, k(f)(IT), were determined to be (3.2 +/- 2.0) x 10(10) M-1 s(-1) and (2.0 +/- 1.3) x 10(10) M-1 s(-1) with the 1,2-DCE vertical bar W (1.5 M Li2SO4) and NB vertical bar W (1.5 M Li2SO4) interfaces, respectively. These values are in the order of the rate constant of diffusion-controlled bimolecular reaction in solution. Also, the formal (standard) potentials of the transfer of [Fe(C5H5)(2)](+) ion at the interface, Delta(W)(O)phi(0)(Fe+), were determined to be -0.015 +/-0.014 V and -0.087 +/- 0.015 V with the 1,2-DCE vertical bar W (1.5 M Li2SO4) and NB vertical bar W (1.5 M Li2SO4) interfaces, respectively. (c) 2006 Elsevier B.V. All rights reserved.