When adsorbed (at a low temperature) on alkali-metal-exchanged zeolites, CO forms both M(CO)⁺ and M(OC)⁺ carbonyl species with the extra-framework alkali-metal cation of the zeolite. Both quantum chemical and experimental results show that C-bonded adducts are characterized by a C-O stretching IR band at a frequency higher than that of 2143 cm^-1 for free CO, while for O-bonded adducts this IR band appears below 2143 cm ^-1. The cation-CO interaction energy is higher for M(CO)⁺ than for M(OC)⁺ carbonyls, although the corresponding difference decreases substantially when going from Li⁺ to Cs⁺. By means of variable-temperature FTIR spectroscopy, this energy difference was determined for several alkali-metal cations, and the existence of a thermal equilibrium between M(CO)⁺ and M(OC)⁺ species was established. The current state of research in this field is reviewed here, with a view to gain more insight into the thermal isomerization process.