Active filler pyrolysis of polymethylsiloxane/aluminum (PMS/Al) mixtures leads to the formation of mullite at 1400 degrees C, and by 1700 degrees C mullite is the only crystalline phase in an amorphous matrix. These mixtures show a complex series of oxidation and reduction reactions prior to the formation of mullite. Initial oxidation of PMS at low temperatures results in the formation of an amorphous product before any significant oxidation of the Al. After melting of the Al phase, redox reactions lead to the formation of elemental Si and SiC as intermediate crystalline phases along with Al2O3 both as the crystalline a-phase and dissolved in the amorphous matrix. Mullite formation occurs by precipitation from the matrix at temperatures above 1400 degrees C. In contrast, PMS/Al2O3 mixtures did not show mullite formation until heat treatment at 1700 degrees C. In this case no significant intermediate crystalline phases occur, except for some limited cristobalite crystallization from the matrix.