Al-filler-loaded polymethylsiloxane (PMS) was pyrolysed in air atmosphere at temperatures 400-1700 A degrees C. The effect of the Al amount added to the PMS on phase development, densification behaviour and microstructure evolution was studied by simultaneous thermal analysis, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The Al/PMS reaction route is complex producing Si, SiO2 (amorphous and cristobalite), Al2O3 (gamma-, iota- and alpha-Al2O3), Al2OC, Al4O4C, Al4SiC4, and AlN, depending on the ratio of Al/PMS in the initial mixture. Increasing the Al content (high Al/PMS ratio) reduces the amount of voids and porosities after PMS degradation. The voids and porosities provide access for the oxygen atmosphere into the inner structure to oxidise the Al particles, Si or SiC and also as channels for the PMS degradation products to escape. Mullite formation was identified in sample containing > 73 wt% Al at temperature as low as 1400 A degrees C.