The in situ polymerization of tetraethoxysilane (TEOS) in poly(vinyl acetate) (PVAc) yields homogeneous and transparent nanocomposite materials. It was shown that covalent bonds occur between the organic chains and the inorganic clusters, preventing macroscopic phase separation. For an initial amount of TEOS higher than a critical value, gel formation is observed. Up to this critical TEOS content; improved mechanical properties, which persist at high temperatures, are obtained.