Epoxy-nanocomposite resins filled with 12-nm spherical silica particles were investigated for their thermal and mechanical properties as a function of silica loading. The nanoparticles were easily dispersed with minimal aggregation for loadings up to 25 wt% as determined using transmission electron microscopy (TEM) and ultra-small-angle X-ray scattering (USAXS). A proportional decrease in cure temperatures and glass transition temperature (for loadings of 10 wt% and above) was observed with increased silica loading. The morphology determined by USAXS is consistent with a zone around the silica particles from which neighboring particles are excluded. The "exclusion zone" extends to 10x the particle diameter. For samples with loadings less than 10 wt%, increases of 25% in tensile modulus and 30% in fracture toughness were obtained. More highly loaded samples continued to increase in modulus, but decreased in strength and fracture toughness. Overall, the addition of nanosilica is shown as a promising method for property enhancement of aerospace epoxy composite resins. Published by Elsevier Ltd.