CO interacts with extra-framework alkali metal cations (M⁺) of zeolites to form both M⁺⋯CO and M⁺⋯OC species. By using variable-temperature FTIR spectroscopy, these C-bonded and O-bonded species were found to be in a temperature-dependent equilibrium. For the same cation, the difference in interaction energy depends upon the zeolite framework. Thus, for the equilibrium process ZNa⁺⋯CO ⇌ ZNa⁺⋯OC, where Z represents the zeolite framework, ΔH⁰ was found to take the values 3.8 and 2.4 kJ mol^-1 for CO/Na-ZSM-5 and CO/Na-Y, respectively. The C-bonded species show always the highest cation-CO interaction energy.