We report the surfactant-free synthesis of new, film-forming vinyl polymer/silica colloidal nanocomposites by copolymerization of 4-vinylpyridine (4VP) with either n-butyl acrylate (n-BuA) or n-butyl methacrylate (n-BuMA) in the presence of an ultrafine aqueous silica sol. Highly transparent, free-standing nanocomposite films were readily obtained by solution-casting from aqueous media at room temperature. Thermogravimetric analyses indicated silica contents ranging from 20% to 56%, depending on the nanocomposite formulation, and intensity-average particle diameters ranged from 139 to 305 run. The influence of the synthesis conditions on the particle properties was also studied. Reducing the initial silica concentration at constant monomer concentration led to an increase in the particle size and reduced colloid stability, indicating that the ultrafine silica sol stabilized the colloidal nanocomposites. Colloidal nanocomposites were also prepared using a methacrylate-capped poly(ethylene glycol) (MPEGMA) macromonomer as a reactive steric stabilizer. The resulting sterically stabilized nanocomposites exhibited enhanced colloid stability, as expected. In addition, this polymeric stabilizer led to an increase in the silica content of the nanocomposites and lower minimum film-forming temperatures due to its plasticizing effect. potential measurements indicated that the MPEGMA stabilizer was mainly located at the surface of the nanocomposite particles, as expected. H-1 NMR spectroscopy studies of the polymeric component extracted from selected nanocomposites confirmed the incorporation of both the 4VP and the film-forming comonomer (and also the MPEGMA macromonomer, where applicable). Transmission electron microscopy studies confirmed the presence of silica in the particles, and thin film cross-sections prepared by cryo-ultramicrotomy indicated that the surface of the nanocomposite particles was silica-rich, suggesting a core-shell morphology. However, X-ray photoelectron spectroscopy studies of the n-BuA-based nanocomposite films indicated that profound morphological changes occurred during film formation. The more hydrophobic copolymer component diffuses to the film/air interface and displaces the hydrophilic silica particles, leading to a surface excess of the copolymer component. This probably accounts for the relatively low water uptake by these nanocomposite films.