Electron correlations within CuO 2 layers, a common structural unit in La-Ba-Cu-O and Y-Ba-Cu-O systems, are studied by using a tight-binding model Hamiltonian. It is found that electron correlations are particularly strong within Cu(3dx2 y2) orbitals. They do not only considerably suppress charge fluctuations, but also change the electron densities. As a consequence~ the average number of holes at Cu sites increases and local moments are formed. Our results agree qualitatively with the experimentally observed magnetic moments in the antiferromagnetic phase and support the point of view that these systems are similar to valence fluctuating systems.