The dynamics of photoinduced heterogeneous electron transfer between a series of ferrocene derivatives and the heterodimer zinc meso-tetrakis(p-sulfonatophenyl)-porphyrin (ZnTPPS4-) and zinc meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP4+) were studied at the polarized water/1,2-dichloroethane interface. The photocurrent responses originating from the heterogeneous quenching of the heterodimer showed a well-defined dependence on the formal Gibbs energy of electron transfer (DeltaG(et)degrees'). The use of various ferrocene derivatives with different redox potentials and potentiostatic control over the Galvani potential difference across the interface allowed modifying DeltaG(et)degrees' over a range of 1 eV. The photocurrent as a function of DeltaG(et)degrees' can be unambiguously described in terms of a Marcus-type behavior of the phenomenological bimolecular electron-transfer rate constant (k(et)(II)). The solvent reorganization energy was estimated to be 1.05 eV, from which an average distance of 0.8 nm between the redox species can be evaluated within the framework of the Marcus model for sharp liquid/liquid boundary. These studies also provided an estimate of the activation-less limit of k(et)(II) of 3 x 10(-19) cm(4) s(-1), which reflects a rather nonadiabatic behaviour of the charge-transfer process. The origin of this nonadiabaticity is connected to the average distance separating the redox species across the interface. Finally, the implications of the observed potential dependence of k(et)(II) current debates about structure and potential distribution across the interface are briefly highlighted.