The conductivity of polynuclear cluster situated between two metallic electrodes is studied within the semiclassical approximation. The conductivity depends on the shape of adiabatic potential surface of 'cluster + excess electron' mixed-valence system which is entirely governed by the relationship between the intramolecular electron transfer and the vibronic coupling. The specific cases of dimer and trimer molecules are illustrated through conductivity and current calculations. It is shown that localization --> delocalization transitions in mixed valence clusters are accompanied by more or less abrupt changes in the molecular conductivity. However, the behavior of molecular conductivity and tunnel current is determined not only by the type of electronic distribution but also by the density of accessible electron-vibrational states. (C) 1998 Elsevier Science B.V.