This paper shows that in copper oxide based compounds where Cu ions are in a square planar geometry with four coordination oxygen ions, i.e. CuO and Nd2CuO4, the Cu2p 3 2 (3d)10L) X-ray photoelectron core lines are narrower than in similar compounds with Cu ions in a pyramidal or octahedral geometry with five or six neighboring oxygens, i.e. Bi2Sr2CaCu2O8 and La2CuO4. To explain this difference two different charge transfer mechanisms from the oxygen p orbitals to the Cu3d orbitals are proposed. For square planar geometry (CuO4) only the px,y → dx2 - y2 charge transfer is allowed, whereas for octahedral or pyramidal geometry the additional pz → dz2 transition could take place. These two different final state configurations will produce different ionization potentials for the Cu2p 3 2 core levels. © 1992.