Four kinds of bridging hydroxyls, (AlO)(3)Si-OH-Al(OSi)(3), (AlO)(2)(SiO)Si-OH-Al(OSi)(3), (AlO)(SiO)(2)Si-OH-Al(OSi)(3), and (SiO)(3)Si-OH-Al(OSi)(3), exist in zeolites NaHX and NaHY. The values of O-H dissociation energies for all kinds of these hydroxyls have been derived from IR spectra (from the values of frequency shifts accompanying the hydrogen-bonding formation with aromatic hydrocarbons and their derivatives). They have also been calculated by the MNDO method by using a series of clusters with various numbers of Al atoms. According to both experimental and theoretical results, the increase in the number of Al atoms close to the bridge decreases the acid strength of bridging hydroxyls. The values of O-H dissociation energies obtained from IR spectra and calculated by the MNDO method agree very well.