Tunneling spectroscopy is a rapidly developing method for obtaining vibrational spectra of monolayers of materials incorporated in tunneling junctions. To date, tunneling spectroscopy has been restricted to observing spectra of junctions that have been warmed to room temperature and exposed to the atmosphere prior to immersion in liquid helium. Here, we report on a method for the in situ preparation of tunneling juncions at low temperatures. This technique avoids questions about atmospheric contamination, and allows the observations of systems too volatile to be otherwise observed. Most of all, due to the low temperature at which the junction is prepared, important mechanisms regarding the change from physiadsorption to chemiadsorption of monolayers on catalytic surfaces may be observed as a function of temperature. In the work discussed here, differential tunneling spectra are shown for the first time on junctions prepared in situ. This has permitted the observation of physioadsorbed CO on aluminum supported cobalt particles with tunneling spectroscopy. © 1982 American Institute of Physics.