The incorporation of lanthanides into polyoxometalates provides entry to new classes of potentially useful materials that combine the intrinsic properties of both constituents. To utilize the [alpha(1)-Ln(H2O)(4)P2W17O61](7-) species in applications of catalysis and development of luminescent materials, the chemistry of this family of lanthanide polyoxometalates in organic solvents has been developed. Organic-soluble polyoxometalate-lanthanide complexes TBA(5)H(2)[alpha(1)-Ln(H2O)(4)P2W17O61] (Ln = La(III), Sm(III), Eu(III), Yb(III)) were prepared and characterized by elemental analysis, acid-base titration, IR, P-31 NMR, and mass spectrometry. The synthetic procedure involves a cation metathesis reaction in aqueous solution under strict pH control. A solid-liquid-phase transfer protocol yielded a unique species (TBA)(8)K-3[Yb(alpha(1)-YbP2W17O61)(2)] with three ytterbium ions and two [alpha(1)-P2W17O61](10-) polyoxotungstates. A centrosymmetric dimeric complex [{alpha(1)-La(H2O)(4)P2W17O61}(2)](14-) was crystallized from aqueous solution and characterized by X-ray diffraction. ESI mass spectral analysis of the complexes TBA(5)H(2)[alpha(1)-Ln(H2O)(4)P2W17O61] shows that similar dimers exist in organic solution, in particular for the early lanthanides. Fragmentation in the mass spectrometer of the complexes from dry acetonitrile solution involves double protonation of an oxo ligand and loss of one water molecule. Low mass tungstate fragments combine into [(WO3)(n)](2-) (n = 1-5) ions and their condensation products with phosphate. Reaction of TBA(5)H(2)[alpha(1)-Eu(H2O)(4)P2W17O61] with 1,10-phenanthroline or 2,2'-bipyridine showed an increase of the europium luminescence. This result is explained by the formation of a ternary complex of [alpha(1)-Eu-(H2O)(4)P2W17O61](7-) and two sensitizing ligands.