The state of copper ion exchanged in ZSM-5-type zeolite has been investigated through the IR and adsorption-heat measurements at 301 K by using CO as a probe molecule. As a result, it was proved that there are at least three kinds of adsorption sites for a CO molecule on the copper ion-exchanged ZSM-5, which are responsible for the IR bands at 2159, 2149, and 2136 cm(-1), as well as the differential adsorption-heats of 91, 82, and similar to 70 kJ mol(-1). Corresponding to these data, TPD also gives three desorption peaks at 363, 442, and 542 K. An excellent linear relationship has been established between the stretching vibrational frequencies due to the adsorbed CO species and the differential adsorption-heat values. When CO gas is introduced to the 723 K-treated sodium ion-exchanged ZSM-5, this sample provides the adsorption-heat of about 35 kJ mol(-1) in the whole adsorption region studied and the weak IR bands at 2175 and 2112 cm(-1). For the sodium ion-exchanged ZSM-5 and CO system, the above correlation does not hold in the same plot containing Cu-CO species. This fact is interpreted in terms of the difference in the nature of bonding between the electrostatic force for the sodium ion-exchanged ZSM-5 and CO system and the covalent nature for the copper ion-exchanged ZSM-5 and CO system. More detailed discussions are also made on the nature of the Cu-CO bond.