CO interacts with Brønsted acid sites of protonic zeolites to form both ZH···CO and ZH···OC hydrogen bonds (Z = zeolite framework); by means of variable-temperature FTIR spectroscopy, these hydrogen-bonded species were found to be in a temperature-dependent equilibrium which, for the faujasite-type H-Y zeolite, involves an enthalpy change of 4.3 kJ mol^-1.