Reaction of Ce(III) with lacunary versions of [H4XW18O62](7-) (X = P, As) yields the 1:2 complexes [Ce(H4XW17O61](19-) (X = As, 1; P, 2) in good yield, characterized in solution and the solid state by NMR spectroscopy and X-ray crystallographic analysis, respectively. The structures confirm a syn C-2 conformation that is analogous to that observed for [Ln(alpha(2)-P2W17O61)(2)](17-) but with "empty" O-4 tetrahedra that are in positions remote from the cerium atom. Bond valence sum calculations for these structures show that the four protons that are required for charge balance in all salts of the XW18 anions and their lacunary derivatives are almost certainly bound to the oxygen atoms of the empty tetrahedra.