Using diffuse reflectance IR spectroscopy, the comparative analysis of absorption bands corresponding to fundamental C=O stretching and combination C=O plus M-C stretching vibrational modes for (CO)-C-12 and (CO)-C-12 + (CO)-C-13 MiXtures adsorbed on metallic Ir in supported 6% Ir/Al2O3 catalyst was performed. For CO adsorbed on metal in linear form the frequencies and intensities of combination absorption bands were insensitive to dipole-dipole coupling between adsorbed molecules. Therefore the carbon-metal bond stretching frequency could be calculated as the frequency difference of the combination mode and the singleton. For the linear form of carbon monoxide adsorption on supported Ir it is equal to 475 cm-1.