The realization of molecular-scale electronic devices will require the development of novel strategies for controlling electrical properties of metallmoleculelmetal junctions, down to the single molecule level Here, we show that it is possible to exert chemical control over the formation of metallmolecule. moleculelmetal junctions in which the molecules interact by pi-stacking The tip of an STM is used to form one contact, and the substrate the other, the molecules are conjugated oligophenyleneethynylenes (OPEs) Supramolecular pi-pi interactions allow current to flow through the junction, but not if bulky tert-butyl substituents on the phenyl rings prevent such interactions For the first time, we find evidence that pi-stacked junctions can form even for OPEs with two thiol contacts. Furthermore, we find evidence for metallmoleculelmetal junctions involving oligophenyleneethynylene monothiols, in which the second contact must be formed by the interaction of the pi-electrons of the terminal phenyl ring with the metal surface