The scanning tunneling microscope (STM) can be used to measure current-voltage characteristics on an atomic scale. The attachment of copper phthalocyanine molecules, in contrast to a variety of other molecules, to graphite changes the electrical characteristics of the STM from relatively symmetric to highly asymmetric or rectifying. Evidence is presented to show that the asymmetry arises because of the electronic energy levels of the copper phthalocyanine. The organic molecules were bonded to the graphite by an acid-base reaction that may have wide applicability.