I.r. spectra with unusually sharp (f.w.h.m.⩽ 5 cm^–1) carbonyl bands and their corresponding ¹³CO satellites follow from the interaction of natural CO with both reduced and oxidized rhodium (1 wt %) introduced into dealuminated zeolite Y (US-Ex). Isotopic exchange with ¹³C¹⁶O, ¹²C¹⁸O, and ¹³C¹⁸O confirms the proposed formation of a well defined dicarbonyl compound with C_2v symmetry and an angle of ca. 106° between the two CO groups. The formation of the dicarbonyl with reduced Rh/US-Ex at 300 K is accompanied by a loss of intensity of the hydroxyl bands of the zeolite, showing the participation of the hydroxyl groups in the formation of Rh^I from small rhodium clusters. The additional intensity increase of the dicarbonyl bands at 423 K proceeds without an intensity loss of the hydroxyl bands and indicates the formation of Rh^I by another process, presumably by CO dissociation. In addition to the carbonyl-stretching bands, combination bands are detected, which allow the estimation of other fundamental bands of the Rh^I(CO)₂ system masked by strong bands of the zeolite framework. During the formation of the Rh dicarbonyl there are no significant changes in the far-i.r. region (400–50 cm^–1) to be assinged to cation motions within the zeolite framework.