The long-term high-temperature cyclic oxidation (100 cycles, 10(4) h, 1500 degreesC) of a Si3N4 material and a Si3N4/MoSi2 composite, both fabricated with Y2O3 as a sintering additive, was studied. Both materials exhibited similar oxidation rates because of surface SiO2 formation described by an almost parabolic law and a total weight gain of 3-4 mg/cm(2) after 104 h. As a consequence of oxidation processes in the bulk, microstructural damage was found in the Si3N4 material. These effects were not observed in the composite. The remarkable microstructural stability observed offers the high potential of Si3N4/MoSi2 composites for long-term structural applications at elevated temperatures up to 1500 degreesC.