1H MAS NMR spectroscopy was applied for a quantitative investigation of the concentration of hydroxyl groups on lanthanum-exchanged zeolites X and Y as a function of the dehydration temperature. A maximum concentration of Brønsted acidic bridging OH groups (SiOHAl) was reached upon dehydration of zeolites La,Na-X and La,Na-Y at 423K. Further raise of the dehydration temperature leads to the recombination of the above-mentioned hydroxyl groups, while the dealumination of the framework is negligible. The maximum number of bridging OH groups formed on zeolites La,Na-X and La,Na-Y correlates well with the lanthanum exchange degree. By adsorption of deuterated acetonitrile as a probe molecule, an increase of the acid strength of bridging OH groups in lanthanum-exchanged zeolites X and Y was found. This finding is explained by the polarizing effect of multivalent lanthanum cations, which leads to a withdrawal of electrons from the O–H bonds of Brønsted acid sites in lanthanum-exchanged zeolites with a high concentration of these extra-framework cations.