Poly(methylsilsesquioxane)(CH3SiO1.5)(n) (PMS) loaded with 40vol.% Si-filler powder was pyrolyzed in inert atmosphere up to 1400degreesC to fabricate Si-O-C composite ceramics. The evolution of the interface microstructure between the filler and the matrix was studied by high resolution electron microscopy (HREM), energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. While below pyrolysis temperatures of 1000 degreesC no filler reaction was observed (inert filler regime), a porous interface layer of nanosized beta-SiC was formed at reaction temperatures above 1200degreesC. Due to a high fraction of open porosity of 13% (1000degreesC) to 19% (1400degreesC) in the polymer-derived Si-O-C matrix, gas-phase transport and reaction processes involving CO and SiO as the dominant species are likely to occur at the interface boundary layer. (C) 2002 Elsevier Science B.V All rights reserved.