The first synthetic pathway using a series of four nonlacunary 4f-heterometal-substituted polyoxotungstate clusters Na-21[(Ln(H2O)-(OH)(2)(CH3COO))(3)(WO4)(SbW9O33)(3)]center dot nH(2)O (NaLnSbW(9); Ln = Tb-III, Dy-III, Ho-III, Er-III, Y-III) as precursors for the directed preparation of nine new 3d-4f heterometallic tungstoantimonates K-5 Na12H3[TM(H2O)-Ln(3)(H2O)(5)(W3O11)(SbW9O33)(3)]center dot nH(2)O (KTMLnSbW(9); TM = Co-II, Ni-II; Ln = Tb-III, Dy-III, Ho-III, Er-III, Y-III) has been developed. Systematic studies revealed an increased K content in the aqueous acidic reaction mixture to be the key step in the cation-directed preparation of 3d-4f compounds; among those, the Cocontaining members represent the first examples of KCoLnSbW(9) (Ln = Tb-III, Dy-III, Ho-III, Er-III, Y-III) heterometallic tungstoantimonates exhibiting the SbW9 building block- All 13 compounds have been characterized thoroughly in the solid state by powder and single-crystal X-ray diffraction (XRD), revealing a cyclic trimeric polyoxometalate architecture with three SbW9 units encapsulating a planar triangle of Ln(III) ions in the case of NaLnSbW(9) and a heterometallic core of one TMII and three Ln(III) for KTMLnSbW(9) (TM = Co-II, Ni-II; Ln = Tb-III, Dy-III, Ho-III, Er-III, Y-III) The results obtained by XRD are supplemented ' for KTMLnSbW(9) by complementary characterization methods in the solid state such as IR spectroscopy, thermogravimetric analysis, and elemental analysis as well as in solution by UV-vis spectroscopy. Detailed magnetic studies on the representative compounds KTMDySbW9 (TM = Co-II, Ni-II) and KCoYSbW9 of the series revealed field-induced slow magnetic relaxation.