Adsorption of CO on CaNaY at 85 K results in formation of three types of calcium carbonyl complexes. At high CO coverage (equilibrium CO pressures above ca. 100 Pa) Ca2+(CO)(3) species, characterized by an IR band at 2185 cm(-1), are predominant. These species are decarbonylated stepwise and first converted into Ca2+(CO)(2) dicarbonyls at decreasing coverage during evacuation. The latter complexes lose one CO ligand at further evacuation and form linear Ca2+-CO species that are quite stable at 85 K. Adsorption of a (CO)-C-12-(CO)-C-13 mixture reveals that the CO ligands in the di- and tricarbonyls behave as independent oscillators. The results obtained are explained by the existence of three coordinative vacancies of Ca2+ cations in SU positions in Y zeolites.