The solid-state ion exchange of Rh chloride with highly dealuminated Y zeolite has been studied by in situ FTIR spectroscopy. Thermal treatment in vacuum and calcination in oxygen up to 600-degrees-C results in an exchange limited to ca. 25% at temperatures higher than 400-degrees-C. The ion exchange can be performed much more effectively (over 50%) and at lower temperatures (100-150-degrees-C) in the presence of CO in the gas phase. A linear correlation has been found between the formation of well-defined Rh(CO)2+ species and the simultaneous loss of acid hydroxyl groups for samples treated at different conditions. From these results a model is proposed with the formation of Rh subcarbonyls, the migration of these species to the cation positions of the zeolite and, finally, the formation of the stable well-defined Rh(CO)2+ species at these positions as essential steps of the ion exchange in the presence of CO.