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.