First-principles calculations were performed on the polarizabilities of noble gas endohedral complexes, X@C-60 ( X = He-Kr). The polarizability of X@C-60 increases from He-Kr, but is distinctly smaller than the sum of the X atoms and C-60, unlike many other non-bonded systems for which an additive rule holds. Compared to C-60, the increased polarizabilities in X@C-60 are mainly from the enhanced charge transfer contribution of the cage and the local polarization of the interior X atoms, though the interior atoms are found less polarized than the corresponding free ones. (c) 2008 Elsevier B. V. All rights reserved.