Twin polymerization was used to prepare composite materials composed of SnO nanoparticles entrapped in a polymer matrix. Novel, well-defined tin-containing molecular precursors, so-called twin monomers, were synthesized by transesterification starting from Sn(OR) (R=tBu, tAm) to give Sn(OCH C H O) (1), [Sn(OCH C H S) ⋅HOCH C H S] (2), [Sn(OCH -2-OCH C H ) ⋅HOCH -2-OCH C H ] (3), [Sn(OCH -2,4-(OCH ) C H ) ⋅HOCH -2,4-(OCH ) C H ] (4), 2,2'-spirobi[4H-1,3,2-benzodioxastannine] (5), 2,2'-spirobi[6-methylbenzo(4H-1,3,2)-dioxastannine] (6), and 2,2'-spirobi[6-methoxybenzo(4H-1,3,2)dioxastannine] (7). C and Sn NMR spectroscopy in the solid state and in solution as well as IR spectroscopy and elemental analysis were used to characterize the tin alkoxides. The molecular structures of compounds 2 and 3 were determined by single-crystal X-ray diffraction analysis. The moisture sensitivity of the tin(IV) alkoxides was demonstrated by the formation of the tin oxocluster [Sn (μ -O)(μ -OH)(μ -OCH C H S) (OCH C H S) (HOCH C H S)] (2 a), a hydrolysis product of compound 2. Polymerization reactions in the melt (for 1 and 5) and in solution (for 2-4) resulted in cross-linked nanocomposites of the type polymer/SnO . Subsequent oxidation of the composites gave SnO with BET surface areas up to 178 m  g . Simultaneous twin polymerization of compounds 5-7 with the silicon derivative 2,2'-spirobi[4H-1,3,2-benzodioxasiline] resulted in the formation of polymer/SnO /SiO hybrid materials. Oxidation gave porous materials with SnO nanoparticles embedded in a silica network with BET surface areas up to 378 m  g . The silica acts as a crystal growth inhibitor, which prevents sintering of the SnO nanoparticles 20-32 nm in size.