It has been shown that SiC whisker-reinforced Al2O3 is susceptible to thermal oxidation because of a series of reactions in which SiC is oxidized and then reacts with Al2O3 to form a whisker-free layer of mullite and silica glass. In this paper, the kinetics of the reaction are analyzed as a function of the composition and whisker content of the composite. The mechanism of the reaction is described in terms of the evolution of phases within the reaction layer on the basis of transmission electron microscope observations and concentration profiles determined by electron microprobe analyses. The reaction is initiated by oxidation of SiC, forming silica glass and graphitic carbon. The silica glass penetrates the Al2O3 matrix along grain boundaries, and reacts to form mullite. As the reaction proceeds, Al2O3 is consumed, leaving residual intragranular Al2O3 islands, and a population of transgranular microcracks in the mullite. It has also been shown that Al2O3-SiC composites are susceptible to crack nucleation and growth under cyclic compressive loads. The presence of the mullite reaction layer on the surface of the composite reduces its resistance to crack initiation under these conditions. The threshold for crack initiation in cyclic compression is measured as a function of reaction layer thickness.