The electron affinity of semiconductors and the work function of metals can be systematically varied by the adsorption of a series of organic molecules with varying dipole moment. This effect can be used to induce molecular control over the electrical characteristics of some metal/semiconductor diodes. The variety of molecules that can be bonded to metals, together with those that are available for bonding to semiconductors, makes the method quite flexible and may enable the build up of more sophisticated interfacial structures.