The integrated absorption coefficient ε has been measured for the stretching band of CO adsorbed on a series of oxides and zeolites. From the temperature dependences of the spectra, ε has been estimated for CO molecules whose stretching frequency is lowered due to their bonding to cationic sites of the zeolite through the oxygen atom. In accordance with the predictions of the electrostatic model, ε decreases as the vibrational frequency of CO increases.