Carbon monoxide interacts with extra-framework K⁺ ions of the zeolite K-ZSM-5 forming both K⁺⋯CO and K⁺⋯OC species, which are characterized by C-O stretching IR bands at 2166 and 2117cm^-1, respectively. By means of variable-temperature FTIR spectrometry, the C- and O-bonded adducts were found to be in a temperature dependent equilibrium which involves a standard enthalpy change of 3.2kJmol^-1. The C-bonded species show the highest cation-CO interaction energy.