We have isolated the 1:1 Ln:[alpha-2-P2W17O61](10-) complexes for a series of lanthanides. The single-crystal X-ray structure of the Eu3+ analogue. reveals two identical [Eu(H2O)(3)(alpha-2-P2W17O61)](7-) moieties connected through two Eu-O-W bonds, one from each polyoxometalate unit. An inversion center relates the two polyoxometalate units. The Eu(III) ion is substituted for a [WO](4+) unit in the "cap" region of the tungsten-oxygen framework of the parent Wells-Dawson ion. The point group of the dimeric molecule is C-i. The extended structure is composed of the [Eu(H2O)(3)(alpha-2-P2W17O61)](2)(14-) anions linked together by surface-bound potassium cations. The space group is P1, a = 12.7214(5) Angstrom, b = 14.7402(7) Angstrom, c = 22.6724(9) Angstrom, alpha = 71.550(3)degrees, beta = 84.019(3)degrees, gamma = 74.383(3)degrees, V = 3883.2(3) Angstrom(3), Z= 1. The solution studies, including W-183 NMR spectroscopy and luminescence lifetime measurements, show that the molecules dissociate in solution to form monomeric [Ln(H2O)(4)(alpha-2-P2W17O61)](7-) species.