Three infrared absorption bands related to fluoride occluded within double four-member ring units of zeolites exist: one corresponds to fluoride itself and the other two are localized vibrations of the cage appearing at a different wavenumber depending on the presence or absence of fluoride within the cage. This is demonstrated by comparison of the vibrational spectra of a series of as-made and calcined octadecasil materials containing varying degrees of fluoride and aluminum and is further supported by ab initio quantum chemical calculations.