In contrast to conventional electrode reactions, an electron transfer (ET) at an oil (O)\water (W) interface is a bimolecular reaction, so that the "microscopic" diffusion of a redox species in the immediate vicinity of an O\W interface should be not a linear one, but like a hemispherical diffusion. Accordingly, the second-order rate constant obtained from usual kinetic measurements involves such a bimolecular-reaction effect, having a certain upper limit determined by the microscopic diffusion process. In this study, the diffusion-controlled rate constant of ET at an O\W interface has been calculated in the analogy of the Smoluchowski-Debye theory for a bimolecular reaction in a homogeneous medium. It has been shown that when the heterogeneous ET process is very fast, the overall or observed rate constant may be restricted by the diffusion-controlled rate constant. (C) 2004 Elsevier B.V. All rights reserved.