Theoretical equations of ac-voltarnmetric current or the equivalent admittance and/or impedance of the electron transfer reaction between a redox couple of A species, A(RED) and A(OX), in organic (O) phase and another redox couple of B species, Box and BRED, in aqueous or water (W) phase at the O vertical bar W interface: A(RED)(O) + B-OX(W) = A(OX)(O) + B-RED(W) are derived when the electron transfer reaction is dc-voltammetric-reversible. The theoretical equations are discussed with reference to the mechanisms by which the electron transfer reaction may take place at the interface, that is, either by the heterogeneous electron transfer reaction mechanism at the interface or by the mechanism in which the homogeneous electron transfer reaction is associated with the ion transfer across the interface. (c) 2007 Elsevier B.V. All rights reserved.