CO adsorption at ambient temperature leads to formation of the well-known Cu+-CO species (2158 cm(-1)) and Cu+(CO)(2) dicarbonyls (2177 and 2150 cm(-1)). In addition, Cu2+-CO complexes (2179 cm(-1)) appear under equilibrium CO pressure and are very easily destroyed by evacuation. The CO stretching frequency almost coincides in position with v(s) of the Cu+(CO)(2) species. Lowering the temperature in the presence of gas-phase CO leads to formation of Cu2+(CO)(2) geminal dicarbonyls (2170 cm(-1)) and a small amount of Cu+(CO)(3) tricarbonyls (evidenced by a band at 2191 cm(-1)). The results demonstrate the high coordinative unsaturation of both Cu+ and Cu2+ ions in Cu-ZSM-5.