Abstract: Ag-mordenite with atomic Si/Al ratio 6.5 was characterized by FTIR spectra of adsorbed CO. Besides H-bonding with surface OH groups and side-on interaction with oxygen atoms of siloxane bridges, adsorption on Ag+ ions occurs, which accounts for the band at 2193–2178 cm–1. The exact position of this band depends on the coverage, it shifts to higher wavenumbers upon desorption, until disappearance after heating in vacuum at 300°C. Frequency growth is accompanied by increase in the integrated absorption coefficient up to 8.0 cm/μmol, which is much higher than that of a free CO molecule. We assume that the changes in band position and absorbance are due to the transition from di- to monocarbonyl, whose bands are not resolved in the fundamental vibration region, but the splitting of overtone band confirms such supposition. The position of the high-frequency component of overtone band is even higher than the double frequency of the fundamental vibration, indicating that the anharmonicity of adsorbed CO is vanished or becomes negative. High intensity of overtone bands infers on nonlinear dependence of molecular dipole on the vibrational coordinate. Spectra registered at elevated temperature do not show bands that could be assigned to O-bonded CO species. Quantum chemical calculation confirms the absence of linkage isomerism for CO bound to Ag+ cations.