text: The adsorption of a very weakly basic or acidic probe molecule can serve to give a picture of Brønsted and Lewis acid–base sites. Infrared spectroscopy of surface sites and adsorbed molecules can then be used to monitor the change in the vibrational frequencies upon adsorption. A very weakly acidic probe molecule can be used to minimize disturbing neighboring sites so that a more accurate measure of surface acidity or basicity can be obtained. A variety of probe molecules can be used including: ammonia, pyridine, acetonitrile, carbon monoxide, and carbon dioxide.