New colloidal vinyl polymer-silica nanocomposite particles of 191 to 333 nm diameter are synthesized via aqueous emulsion polymerization of either styrene or a 1:1 styrene/n-butyl acrylate mixture at 60oC using a cationic azo initiator in the presence of a surface-functionalized ultrafine aqueous silica sol as the sole stabilizing agent. Unlike previous formulations, this new protocol is surfactant-free and requires neither auxiliary comonomer nor any non-aqueous co-solvent. Optimization of the silica sol concentration leads to remarkably high silica aggregation efficiencies for both polystyrene/silica and poly(styrene-stat-n-butyl acrylate)/silica nanocomposite syntheses. The latter formulation allows highly transparent films (containing up to 38 wt. % silica) to be cast at ambient temperature. Thus this formulation offers considerable potential for tough, transparent nanocomposite coatings. Chemical degradation studies (either by calcination to pyrolyze the polymer or by concentrated NaOH to remove the silica) suggest that these new nanocomposite particles have ‘core-shell' particle morphologies.