FTIR spectra of CO adsorbed on reduced Ni-containing USY zeolites with Si/Al=30 besides weak H-bond with bridged hydroxyls and silanol groups and bonding to Ni2+ cations reveal formation of mono-, diand tricarbonyls with Ni+ sites. The bands of adsorbed CO species are very narrow, with FWMH~1—1.5 cm−1. The spectra of isotopically mixed nickel dicarbonyl species reveal the splitting, which shows that two CO molecules occupy non-equivalent positions, so that the species exist in two isomeric forms, despite the similarity of sites, following from the spectra of monocarbonyls. Possible explanations of this phenomenon are discussed.