Silica nanoparticles having specific surface area (wSSA) 50-300 m(2) g(-1) were admixed into vinyl-terminated dimethylsiloxy monomer with a dual asymmetric centrifuge (planetary mixer) and cured to form PDMS-based nanocomposites containing up to 12 vol% SiO2. Thin sections of cured nanocomposites obtained by a cryostate-microtome were analyzed by TEM while small and ultra small angle X-ray scattering (U/SAXS) was used to determine nanocomposite structure: filler primary particle, aggregate (chemically or sinter-bonded particles) and agglomerate (physically-bonded particles) size as a function of mixing duration and filler concentration. More aggregated silicas with higher SSA exhibited denser crosslinking than less aggregated ones regardless of crosslinker content as determined by swelling nanocomposites in toluene at equal filler content. The nanocomposite strength was determined by tensile tests (Young's modulus and elongation at break). Consistent with "bound rubber" theory, the Young's modulus of the nanocomposites increased non-linearly with increasing filler volume fraction. (C) 2010 Elsevier Ltd. All rights reserved.