The oxidation behaviour of attrition-milled Al88Si12/Al2O3 powder mixtures was investigated for the formation of mullite/Al2O3 composites by the reaction bonded alumina (RBAO) process. Cylindrical powder compacts were heated at 5-degrees-C min-1 to temperatures between 450 and 1400-degrees-C. Oxidation occurred rapidly between ca. 400 and 750-degrees-C. Dense, outer reaction layers which formed at the lower temperatures inhibited complete oxidation and led to fracture of the body during continued heating to higher temperatures (above ca. 850-degrees-C) While the incorporation of ZrO2 improved the oxidation of the samples, X-ray analysis indicated that the Si in the alloy reacted with the ZrO2 to form phases which prevented the formation of mullite at the temperatures used in the experiments.