1H MAS NMR spectroscopy was applied for the quantitative investigation of the concentration of hydroxyl groups in zeolites La,Na-X with different lanthanum-exchange degrees as a function of the dehydration temperature. A maximum concentration of Brønsted acidic bridging OH groups (SiOHAl) corresponding to ca. 1 OH per lanthanum cation was reached upon dehydration of zeolites La,Na-X at 423 to 473 K. Further raise of the dehydration temperature led to a dehydroxylation of the above-mentioned zeolites, while the dealumination of the framework was negligible. By adsorption of deuterated acetonitrile as probe molecule, a significant increase of the acid strength of bridging OH groups was found with increasing lanthanum exchange degree. This finding is explained by the polarizing effect of the multivalent lanthanum cations, which leads to a withdrawal of electrons from the O-H bonds of Brønsted acid sites in zeolites La,Na-X possessing a high concentration of these multivalent extra-framework cations.