When adsorbed on the faujasite-type K-Y zeolite, carbon monoxide forms cationic C-bonded K(CO)+ carbonyls and also O-bonded K(OC)+ isocarbonyls. The carbonyl species gives a characteristic C-O stretching band at 2158 cm−1, while that of the isocarbonyl appears at 2124 cm−1. By means of variable-temperature infrared spectroscopy, these K(CO)+ and K(OC)+ species were found to be in a temperature-dependent isomerization equilibrium, where the relative proportion of the isocarbonyl increases as temperature is raised. The corresponding isomerization enthalpy resulted to be 1.6(±0.5) kJ mol−1. Comparison with data available for other alkali-metal-exchanged zeolites shows that, for most cases, this thermodynamic parameter is an approximately linear function of the electrostatic field at the cation site. Lithium-exchanged zeolites, however, do not seem to conform to this rule; presumably because of the strong polarizing power of the Li+ ion.