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Composites of poly (vinyl acetate) (P V Ac) and silica were prepared from the in situ polymerization of acid-catalyzed tetraethoxysilane. The physical properties of these composites were compared with those of PVAc/fumed silica (of 7-nm nominal particle size) composites prepared by both melt-milling and solutioncasting from tetrahydrofuran or carbon tetrachloride. Their morphologies were examined by transmission electron microscopy and small-angle X-ray scattering revealing that, although all the materials were optically transparent, differences in the primary particle size and interfacial regions exist. Infrared spectroscopy showed that substantially more hydrogen bonding occurs in the in situ prepared composites. The dielectric, dynamic mechanical, and ultimate mechanical properties of the composites indicated that a connected, supporting silicate network phase is formed at much lower concentrations of silica in the in situ prepared composites. This is shown to have a substantial effect on the tensile properties of the composite at temperatures above the glass transition temperature (Tg) of PVAc but does not dramatically influence these properties below Tf.