Abstract I.r. spectra of NaH-Y zeolite shows two bands of OH stretching frequencies around 3550 and 3650 cm −1, the latter being composed of 3648 and 3660 cm −1 peaks indicating the presence of two kinds of high frequency OH groups. They readily interact with pyridine to form two kinds of pyridinium ions with deformation vibrations at 1543, 1627 cm −1 and 1552, 1636 cm −1 respectively. The same pyridinium ions are observed on NaMg-Y, NaCo-Y and Na-Y zeolites after adsorption of water; which generates Brønsted acidity by the reaction: Me 2+(H 2O) n → Me(OH) + + H +, the localization of the two kinds of OH groups being independent of the exchanged cation. Pyridine coordinately adsorbed on cations is visible, the adsorption strength decreases in the order: Co 2+> Mg 2+ > Na +.