The reaction of mid-lanthanide (Ln) ions with the preformed {Se6W39} precursor under reasonably acidic aqueous conditions in the presence of organic amine cations results in an unprecedented nanoscale lanthanide-functionalized polyoxotungstate family, which are rare examples of mid-lanthanide-containing selenotungstates. (C4H10NO)(9)Na-3-[Dy3Se3.5W30O107.5 (H2O)(10)]center dot 22H(2)O (1) and (NH4)(3) (C2H8N)-Na-2[Dy4Se6W38O132(H2O)(26)(OH)(6)]center dot 18H(2)O (2) reveal a trimeric Keggin assembly and a cyclic {Se6W38}-based chain, respectively, whereas (NH4)(4)Na-8[Gd4Se6W48O166(H2O)(20)(OH)(4)]center dot 21H(2)O (3) and (NH4)(9) (C2H8N)(4)Na-5[Ln(6)Se(6)W(58)O(202)(H2O)(20)(OH)(4)]center dot 58H(2)O (4; Ln = Gd, Tb, or Dy) are a few examples of polyoxometalates consisting of both classical Keggin and Wells-Dawson building blocks, and (NH4)(4) (C2H8N),Na-13[Ln(4)Se(8)W(56)O(196) (H2O)(x) (OH)(1)(0)]center dot 40H(2)O (5; Ln = Gd, Tb, or Dy; x = 12 for Gd and Tb and 10 for Dy) features the largest "pure" Wells-Dawson selenotungstate {Se8W56} bearing a length of 3.73 nm. A library of Se-templated species involving the first reported Keggin {alpha-SeW8} and Wells-Dawson {alpha-Se2W16} building blocks as well as some decisive assembly factors during the synthesis is responsible for these architectures. All of the compounds were structurally characterized in the solid and solution by single-crystal X-ray diffraction, IR, thermogravimetric-differential thermal analysis, and electrospray ionization mass spectrometry. Magnetic properties indicate that 1 and 4-Dy show probable single-molecule-magnet behavior with obvious frequency dependence, whereas 3 and 4-Gd present the antiferromagnetic interactions between the GP(III) centers.