This contribution describes a simple model of a molecular interferometer. We consider an electron transfer molecule consisting of electron-donating and electron-acceptor parts that are connected by a bridge. The bridge provides two parallel tunneling pathways for electron transfer from donor to acceptor. In each pathway the electron can interact with a pathway-specific bridge vibration. We study electron transfer from donor to acceptor mediated by inelastic tunneling, where the tunneling electron exchanges energy with the local bridge vibrations. When the tunneling electron excites or is excited by a bridge vibration specific to one of the tunneling pathways, the tunneling pathway is "labeled" and the interference between tunneling pathways is lost. We discuss types of molecules and optical-spectroscopy experiments which could allow the observation of this effect. © 2007 American Institute of Physics.