A composite consisting of 30 wt% SiC whiskers and a mullite-based matrix (mullite-32.4 wt% ZrO2-2.2 wt% MgO) was isothermally exposed in air at 1000 degrees-1350 degrees C, for up to 1000 h, Microstructural evolution in the oxidized samples was investigated using X-rap diffractometry and analytical transmission electron microscopy. Amorphous SiO2, formed through the oxidation of SIC whiskers, was devitrified into cristobalite at T greater than or equal to 1200 degrees C and into quartz at 1000 degrees C. At T greater than or equal to 1200 degrees C, the reaction between ZrO2 and SiO2 resulted in zircon, and prismatic secondary mullite grains mere formed via a solution-reprecipitation mechanism in severely oxidized regions. Ternary compounds, such as sapphirine and cordierite, also mere found after long-term exposure at T greater than or equal to 1200 degrees C.