Si-filler-loaded poly(methylsilsesquioxane) was pyrolyzed in nitrogen atmosphere between 1250 and 1600degreesC to fabricate Si-O-C-N micro-composite materials. The evolution of the microstructure and the resulting nanocomposition were studied by X-ray diffraction, scanning electron microscopy (SEM), high-voltage electron microscopy, high-resolution electron microscopy, electron energy-loss spectroscopy (esp. energy-loss near-edge structures), and porosity analysis. At pyrolysis temperatures below the melting point of Si (1414 degreesC) the silicon oxynitride (Si2ON2) formation proceeds via a vapor phase reaction between SiO and N-2 in the pores and results in fiber-like aggregates with distinct core-shell structures containing nanocrystalline regions. At higher temperatures the molten Si filler is involved in the silicon oxynitride formation reactions, and above 1500degreesC a complete conversion of the Si filler and the Si-C-O matrix into Si2ON2 takes place with a significant reduction of the porosity in comparison to the gas phase reaction regime. (C) 2003 Elsevier Science B.V. All rights reserved.