CO interacts with exchangeable cations M⁺ (gray spheres in the picture) of zeolites to form M⁺ ··· CO and M⁺ ·· OC species (C: black; O: white) which are in a temperature-dependent equilibrium. For Na-ZSM-5 (M⁺ = Na⁺) the difference in interaction energy amounts to 3.8 kJ mol^-1, as determined by means of variable-temperature FT-IR spectroscopy.