Reaction of (C6H5)SnCl3 with Na-10[A-alpha-GeW9O34] in water results in the monomeric, trisubstituted Keggin species [{(C6H5)Sn(OH)}(3)(A-alpha-GeW9O34)](4-) (1), constituting the first organotin derivative of a trilacunary Keggin tungstogermanate. Polyanion 1 could be obtained as two different cesium salts depending on the applied isolation strategy: Cs3Na[{(C6H5)Sn(OH)}(3)(A-alpha-GeW9O34)]center dot 9H(2)O (CsNa-1) and CS3[{(C6H5)Sn(OH)}(3)(A-alpha-HGeW9O34)]center dot 8H(2)O (Cs-H1). The monomeric phenyltin-containing tungstosilicate [{(C6H5)Sn(OH)}(3)(A-alpha-SiW9O34)](4-) (2) and the dimeric, sandwich-type derivative [{(C6H5)Sn(OH)}(3)(A-alpha-H3SiW9O34)(2)](8-) (3) have also been isolated as the cesium salts CS3Na[{(C6H5)Sn-(OH)}(3)(A-alpha-SiW9O34)]center dot 9H(2)O (CsNa-2), CS4[{(C6H5)Sn(OH)}(3)(A-alpha-SiW9O34)]center dot 13H(2)O (Cs-2), and CS8[{(C6H5)Sn(OH)}(3)(A-alpha-H3SiW9O34)(2)]center dot 23H(2)O (Cs-3), respectively. We have investigated in detail the similarities and differences in the reactivity of (C6H5)Sn3+ with [A-alpha-GeW9O34](10-) vs [A-alpha-SiW9O34](10-). All five compounds have been characterized in the solid state by means of elemental analysis, infrared spectroscopy, thermogravimetry, and single-crystal X-ray diffraction, representing the first structural analysis for polyanions 1-3. A full solution characterization of 1 by multinuclear NMR spectroscopy (H-1, C-13, Sn-119, and W-183) has also been performed. The monomeric polyanions 1 and 2 are closely associated in the solid state through (Sn)O-H...O-t (O-t: terminal oxygen atom) hydrogen bonds reinforced by weak C-H...O-t contacts to form 2-dimensional (CsNa-1 and CsNa-2) or 1-dimensional (Cs-H1) arrangements, and also dimeric entities (Cs-2) depending on the network of intermolecular interactions.