The ionization energies of Eu-n(COT)(n) (n = 1-4) were found to decrease asymptotically with the cluster size. The low-spin state X and the high-spin state a were characterized with DFT calculations; their adiabatic energy difference was found to decrease with the cluster size. The calculated ionization energies of the X states reproduced the experimental size dependence. Those of the a states also agreed except for n = 1. These features were explained by the significant variation of the electrostatic potential at ionization sites. The state-specific permanent dipole moments were calculated to clarify the difference in the electronic states. (C) 2005 Elsevier B.V. All rights reserved.