A family of solution-stable polyanions [Na�{Ln III -(H 2 O)}{W VI O(H 2 O)}P V 4 W VI 26 O 98 ] 12À (Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) represent the first examples of polyoxometalates comprising a single lanthanide(III) or yttrium(III) ion in a rare trigonal prismatic O 6 environment. Their synthesis exploits the reactivity of the organophospho-nate-functionalized precursor [P 4 W 24 O 92 (C 6 H 5 P V O) 2 ] 16À with heterometal ions and yields hydrated potassium or mixed lithium/potassium salts of composition K x Ln y H 12-x-y [Na�{Ln-(H 2 O)}{WO(H 2 O)}P 4 W 26 O 98 ]•nH 2 ; y = 0-2; n = 24-34; m = 0-1.5). The Dy, Ho, Er and Yb derivatives are characterized by slow magnetization relaxation.