Addition of CO on Cu-exchanged zeolite was investigated by means of quantum chemical calculations based on density functional theory. The aim of this investigation was to get insights about changes of electronic properties of a copper site with zeolite composition by using a CO probe molecule. Calculated v(CO) frequency values show that various Si/Al ratios of faujasite zeolite reproduce the expected experimental decrease of the v(CO) values with decreasing Si/Al ratio. These calculations predict that H/Na ratio variations also induce changes in the v(CO) values. These results illustrate that different compositions of the zeolite change the electronic properties of copper that are reflected in the v(CO) frequency values. DFT results showed also that different structures and CO adsorption energies are obtained due to various Si/Al and H/Na ratios of the zeolite. Finally, these calculations evidence the possibility for CO to be connected at the same time to Cu-I and to a close Na cation, Cu being at site II and Na at site II in Cu-I-exchanged faujasite. A DRIFT experiment on two samples of faujasite, Cu( 28) H( 51) NaY and Cu( 25) H( 0) NaY, supports v(CO) displacements to higher energy values with increasing H/Na ratio.