The interaction of alkenes, acetylene, benzene and acetone with Cu+ sites in Cu-zeolites resulted in the activation of the multiple bond. The bond weakening was pronounced in the case of alkenes and acetylene, less distinct in acetone only slightly marked in benzene. The present study was undertaken to follow the effect of interaction of organic molecules with Cu+ sites on the properties of these Cu+ ions, as well as the effect of interaction with CO on the interaction of the same Cu+ with organic molecules. IR experiments evidenced that organic molecules replaced CO from relatively stable Cu+-CO complexes. This can be related to the distinctly higher energy of bonding of Cu+ with organic molecules than with CO. In the case of zeolites CuY and CuX there are several kinds of Cu+ of various electron donor properties. Organic molecules react preferentially with less electron donor Cu+, suggesting that pi donation has more important impact on the strength of the bonding of organic molecules with Cu' than pi back donation. On the other hand, pi back donation has more impact on the activation of the multiple bond. At relatively high CO pressure one Cu' ion can bond simultaneously CO and organic molecule. Cu+ ion, zeolite framework, organic molecule and CO act as one system. CO acts as a strong electron acceptor and the presence of organic molecule increases the electron flow towards CO, enhancement of IT back donation to antibonding pi* orbitals of CO and therefore weakening of C equivalent to O bond occurs. On the other hand, the electron flow toward organic molecule is slighter than without CO, what results in a smaller weakening of C-C bond in benzene and C=O in acetone. (c) 2006 Elsevier B.V. All rights reserved.