The presence of TiC or TiN paritcles in an Al2O3matrix affects the thermal stability of the composites in oxidizing environments. In isothermic oxidation tests at 700°, 800°, 900°, 1000°, and 1100°C for up to 20 h, two different oxidation regimes have been observed atT<900°C and at 900°C ≤T≤ 1100°C. At low temperatures (T<900°C), the oxidation follows a phase‐boundary reaction; the reaction product initially consists of aggregates of submicrometer needlelike TiO2rutile crystals that subsequently grow and coalesce. When a continuous TiO2rutile layer is formed (T≥ 900°C), the oxidation kinetics change to parabolic, and the diffusion of O2through a thick TiO2layer is proposed as t