The effect of temperature (300-673 K) on the acid-bridged OH groups of mordenite zeolite with Si/Al ratios between 5.56 and 23.60 was investigated by in situ IR spectroscopy. With increasing temperature, the frequencies of acid-bridged OH groups are shifted to lower values and shape of the spectrum becomes broad. The sensitivity of nu(OH) of the samples to the temperature shows differences (especially above 473 K). Correspondingly, the acidity of the sample shows differences at different temperatures and increases with increasing temperature. Moreover, some weaker acid sites can be made stronger as the temperature rises. In a series of mordenite tested, in the lower temperature region (300-473 K), the order of acidity depends on the conditions of treatment. At higher temperatures (473-673 K), with increasing temperature the acidity of the sample decreases, reaches a minimum (Si/Al = 11.18), and increases again (Si/Al > 11.18).