Model clusters of a Cu(II)-Y zeolite have been studied using a density functional theory based method in order to investigate the electronic properties of the metal site involved in the catalytic activity of this zeolite. This work has shown that different Si/Al ratios, sizes, and shapes of the models do not induce significant changes in the electronic propel-ties of the Cu site, whereas the influence of its coordination is emphasized. A large charge transfer (CT) from the zeolite to the metal has been found in the case of a formal Cu(II) cation. Comparable net charges on Cu are thus obtained for models of Cu(II) and Cu(I)-zeolite. However, these two systems differ by their frontier orbital patterns, which are characterized by a low lying empty orbital localized on the zeolite and on Cu in the case of Cu (II), whereas the lowest virtual molecular orbital of the Cu(I) model has a much higher energy. Addition of NH3 molecules to the Cu(II) model is accompanied by a rearrangement in the electron distribution of the whole system, underlying the non negligible role of the zeolite ill the catalytic process.