Silica nanoparticles for polymer nanocomposites are made by oxidation of hexamethyldisiloxane (HMDSO) in methane/oxygen diffusion flames. The flame temperature is measured by in-situ Fourier transform infrared (FTIR) spectroscopy while the degree of agglomeration of the product powder is quantitatively determined by ultra small angle X-ray scattering (USAXS) and is confirmed by transmission electron microscopy (TEM). Precisely controlled, non-agglomerated silica particles having an average primary particle diameter of 18-85 nm, as determined by N-2 adsorption and TEM, are made at low silica production rates of 9 g/h or at low O-2 flow rates at silica production rates of 17 g/h, while smaller and highly agglomerated particles are made at high O-2 flow rates at silica production rates of 17 g/h. The differences in morphology result from the completion of gas-to-particle conversion and from the onset of steep cooling in the flames that determines the duration of full coalescence. Nanocomposites with dimethylacrylate polymers are made using non-agglomerated silica particles and compared to the ones made with commercially available silicas. (C) 2004 Elsevier B.V. All rights reserved.