Fourier-transform IR spectroscopy was used to study the interaction of CO with the acidic hydroxyl groups in H-mordenite and with extraframework cations in alkali-metal exchanged X-mordenites (X = Li, Na, K, Rb, Cs). The dipolar CO molecule was found to interact specifically with extraframework metal ions located at two different sites, the main channels and the side pockets. For cations inside the main channels of the zeolite, the C-O stretching frequencies increase continuously from 2155 cm(-1) for Cs+ up to 2188 cm(-1) for Li+. Cations located at the bottom of the side pockets were found to induce a smaller shift from the frequency corresponding to the free CO molecule (<(nu)over tilde>(CO) = 2143 cm(-1)). The observed frequency shifts were used to evaluate the electrostatic field (inside the main channel) created by the extraframework cations. Values in the range of 2-8 V nm(-1) were obtained. Nonspecifically adsorbed (physisorbed) CO was also observed, which gave rise to a complex IR absorption band at about 2138 cm(-1).