Adsorption of pyridine on zeolites can show three modes of sorption: physisorption, sorption on the LF hydroxyls and sorption on the HF hydroxyls. In this study a separate IR spectrum was obtained for each of the three sorbed pyridine species and major differences were observed, especially in the ring-bending and stretching region (2000 to 1200 cm^–1). A comparison of the spectra showed that pyridine sorbed on the LF hydroxyls is not physisorbed, but protonated to some degree. H/D exchange enabled the IR spectrum of the bonded Brønsted proton to be obtained for pyridine sorbed on the HF hydroxyl sites which proved to be a broad band from ca. 3600 to ca. 2600 cm^–1. For pyridine sorbed on the LF hydroxyls, a broader more complex band was observed from ca. 3700 to ca. 2200 cm^–1. Reversible sorption/desorption occurred for physisorbed pyridine and pyridine sorbed on the LF hydroxyls, but complete desorption of pyridine from the HF hydroxyls resulted in loss of acid sites.