The sodium ion in the heteropolytungstate known as the Preyssler anion, [NaP5W30O110](14-), was exchanged with cerium from aqueous solutions of ammonium eerie nitrate, [NH4]Ce-2(IV)(NO3)(6), as described by Creaser et al. [Inorg. Chem. 1993, 32, 1573]. The valence of cerium in this heteropolyanion was determined through Ce L-edge XANES, X-ray absorption near-edge structure. The XANES results demonstrate that cerium is trivalent in the Ce-exchanged Preyssler heteropolyanion in the solid state and in aqueous solution (1 M H2SO4) at rest potential and after constant-potential, bulk electrolysis at -0.55 V vs SCE. The encapsulated sodium ion of the Preyssler anion was shown to be directly exchangeable with Ce-III by prolonged (48 h), high temperature (165 degrees C) aqueous treatments with either Ce(NO3)(3).6H(2)O or CeCl3.7H(2)O in Teflon-lined pressure vessels.