Eight tris(organotin)-substituted Keggin tungstosilicate heteropolyanions have been synthesized and characterized by elemental analysis, infrared and Mossbauer spectroscopy, multinuclear NMR, and X-ray crystallography. The new anions contain alpha- or beta-SiW9O3410- moieties and are of two structural types, [(RSn)(3)(SiW9O37)](7-) (R, isomer : Ph, alpha-, 1; n-Bu, alpha-, 2; Ph, beta-, 3; n-Bu, beta-, 4) and [(RSnOH)(3)(SiW9O34)(2)](14-) (Ph, alpha-, 5; n-Bu, alpha-, 6; Ph, beta-, 7; n-Bu, beta-, 8). Crystals of Cs4H3[(PhSn)(3)(SiW9O37)]. 8H(2)O (anion 3) are monoclinic, space group C2/c, with lattice constants a = 48.91(2) Angstrom, b = 12.111(3) Angstrom, c = 20.334(9) Angstrom, beta = 102.30 degrees, and Z = 8. The anion has nominal C-3v symmetry and has a structure with three comer-shared WO6 octahedra of the beta-Keggin anion replaced by three PhSnO(5) groups. Crystals of Cs9H5[(BuSnOH)(3)(SiW9O34)(2)]. 36H(2)O (anion 6) are tetragonal, space group P (4) over bar 2(1)m, with lattice constants a = b = 29.005(4) Angstrom, c 13.412(4) Angstrom, and Z = 4. The anion has the anticipated D-3h symmetry and contains three BuSnOH groups sandwiched between A,alpha-SiW9O3410- anions.