The adsorption of pyridine in NaH-mordenites of various Na/H exchange degrees was followed by adsorption microcalorimetry and IR spectroscopy. IR data evidenced that practically only Bronsted acid sites (OH groups) were present in our mordenites, The concentration of Lewis sites was very low. All the results obtained in this study concern therefore the properties of acid hydroxyls in large channels, which are active sites in reactions catalyzed by mordenites. The amounts of such hydroxyls determined volumetrically were found to be close to the amounts determined in a previous IR study. The average acid strength determined microcalorimetrically increased with exchange degree up to 47% and then was constant. It indicates that as long as new hydroxyls are formed in large channels, their average acid strength increases (probably more strongly acid hydroxyls are formed at higher exchange degrees). Above 47% of exchange, the acid strength of hydroxyls in large channels was constant, indicating that the formation of hydroxyls in side pockets (which takes place above 50% of cation exchange) did not influence the properties of hydroxyls in large channels. The differential heat of pyridine adsorption with most of acid sites (OH groups) did not depend on pyridine loading. Our earlier IR and TPD results of ammonia desorption suggested heterogeneity of OH groups in large channels of mordenites. However, the constant value of adsorption heat suggests that pyridine molecules reacted with all the accessible hydroxyls without selecting the most acid ones. This hypothesis was confirmed by IR studies of pyridine adsorption : the stretching frequency of hydroxyls consumed by pyridine was independent on the amount of pyridine adsorbed.