The ammonium salt of the 1:1complex (1) of Ce(III) with alpha (1)[P2W17O61](10-) was prepared and characterized by elemental analysis, vibrational and NMR spectroscopy (P-31, W-183) cyclic voltammetry, and single-crystal X-ray analysis (P (1) over bar; a 15.8523(9) Angstrom, b = 17.4382(10) Angstrom, c = 29.3322(16) Angstrom, alpha = 99.617(1)degrees beta 105.450 (1)degrees, y = 101.132(1)degrees, V = 7460.9(7) Angstrom, Z = 2). The anion consists of a centrosymmetric head-to-head dimer, [{Ce(H2O)(4)(P2W17O61)}(2)],(14-) with each 9-coordinate Ce cation linked to four oxygens of one tungstophosphate anion and to one oxygen of the other anion. On the basis of P NMR spectroscopy, a monomer-dimer equilibrium exists in solution with K = 20 +/- 4 M-1 at 22 degreesC. Addition of chiral amino acids to aqueous solutions of 1 results in splitting of the P-31 NMR signals as a result of diastereomer formation. No such splitting is observed with glycine or DL-proline, or when chiral amino acids are added to the corresponding complex of the achiral alpha (2)-isomer of [P2W17O61](10-) From analysis of the P-31 NMR spectra, formation constants of the two diastereomeric adducts of 1 with L-proline are 7.3 +/- 1.3 and 9.8 +/- 1.4 M-1.