A detailed FTIR characterization of acid sites in two structurally related zeolites, ferrierite and SUZ-4, is carried out. Assignment of OH groups observed in the spectra is addressed taking into account zeolite structure and composition. In ferrierite, at least four types of bridging OH groups vibrating in 10-, 8-, and 6-membered rings are identified using a combination of experimental and computational techniques. Our results suggest that framework aluminum in ferrierite is preferentially sited in extended rings at the intersections of 8- and 6-ring channels resulting in a high concentration of OH groups in the large cages in 8-ring channels. The IR band corresponding to these hydroxyls, 3601 cm(-1), is not significantly shifted to low frequencies because the OH groups are vibrating in extended rings. At the same time, access to these hydroxyls is restricted by 8-membered rings. The band at 3565 cm(-1) assigned to the bridging OH-groups in 6-membered rings is responsible for the strongly asymmetric shape of the experimental spectrum. For SUZ-4 zeolite, deconstruction of the spectrum, supported by H-D exchange and accessibility studies, results in three IR bands at 3560, 3592, and 3610 cm(-1) corresponding to OH groups in 6-, 8- and 10-membered rings. The intensity of the first band is considerably lower than that in ferrierite, indicating that the residual potassium cations are located in small cages or in double 6-rings. The low-intensity band at 3560 cm(-1) and the similar intensities of the bands at 3592 and 3610 cm(-1) lead ro a near-symmetrical experimental spectrum.