We have performed electroreduction of bisthienyl aminobenzene and thienyl aminobenzene diazonium salts on glassy carbon and polycrystalline gold and generated ultrathin organic films. This method allows covalent bonding between the corresponding aromatic radical and the electrode. Electrochemical behaviors of these modified electrodes toward reversible outer-sphere redox species show unusual properties. No current is observed in the potential range where redox probe reactions on bare electrodes usually occur. The organic layer totally blocks the electrode in such a potential window. Above a threshold voltage, the current dramatically increases and an "irreversible" wave is observed. These results reveal that ultrathin organic layers constitute a reversible conductance switch. Moreover, the threshold voltage of this organic layer can be tuned by nature of the pi-conjugated molecules grafted onto the electrode. Such junctions based on conjugated oligomers with a well-defined metal/oligomer interface retaining reversible on/off switching capabilities controlled by the redox state of the oligomer will be of interest as active interconnects for molecular electronics or in redox molecular actuators.