By in-situ FTIR study using CO as a probe molecule, a new type of Cu+ site, identified as defective (AlOdelta-)Cu+, was found to exist mainly in activated solution-exchanged Cu-ZSM-5, and was thought to derive from mixed ore-bridged Cu+ ions, (SiAlO)Cu+, due to high-temperature (400 degrees C) calcination. At room temperature the CO adsorption on this sample produced three infrared carbonyl bands at 21761 2156, and 2138 cm(-1). The first two bands, previously ascribed to the CO adsorbed at CU2+ sites and mixed ore-bridged (SiAlO)Cu+ ions, respectively, Vanished upon Ar purge at room temperature whereas the last band remained unaffected. The 2138 cm(-1) band was assigned to the CO adsorbed at defective (AlOdelta-)Cu+ site, and its desorption energy was higher than those at ore-bridged Cu+ and Cu2+ sites. The stronger Cu+-CO bonding and weaker C-O stretching are explained by a stronger pi back-donation due to the defective structure. The 2138 cm(-1) band was found to convert to the 2156 cm(-1) band during heating between 100 and 280 degrees C, with an estimated activation energy of the transition approximately equal to 4.8 kcal/mol. A reaction scheme is proposed to depict the mechanism of this transition.