Although the remarkable structural and surface properties of those crystalline aluminosilicates classified as zeolites have been known for some considerable time, it is only quite recently that their catalytic properties have been recognized and utilized. The properties of zeolites as catalysts have been recently discussed in detail in this journal by Turkevich [1]. Many review articles give details of their structural [2–5] and surface [6–9] properties, as studied by classical methods. This review will be restricted to information on zeolites obtained by applying one of the newer methods of studying the solid-gas interface-infrared spectroscopy.