Poly(dimethylsiloxane)(PDMS)-metal-oxo nanocomposites have been prepared as transparent monoliths from dimethyldiethoxysilane (DMDES) and metal alkoxides, M(OR)n, where M ) Al(III), Ge(IV), Sn(IV), Ti(IV), Zr(IV), Nb(V), and Ta(V). An accurate structural analysis of these hybrid materials has been performed by FTIR, Raman, and multinuclear NMR (1H, 13C, and 29Si) spectroscopies, SAXS, and DSC measurements. These techniques show that all the hybrid systems present a structure based on amorphous metal-oxo nanodomains embedded within the siloxane network. However, a strong influence of the cross-linking metal nature on the size of the metal-oxo nanoparticles and on the extent of the interface between inorganic domains and the siloxane component has been found. Spectroscopic measurements reveal a more important nanophase separation for the PDMS systems incorporating Al(III), Ti(IV), or Zr(IV) species as cross-linking agent than for the ones crosslinked with Nb(V), Ta(V), or Ge(IV) (in which 45% of the silicon atoms are spatially near a metal species).