The present study aims to investigate the oxidation behaviour of an AlN-SiC composite, pressureless sintered with the addition of Y2O3. Two main aspects are considered: (1) the evaluation of the oxidation kinetics in the temperature range 1300-1450degreesC for short term tests (30 h) and (2) the degradation of the flexural strength after oxidation at temperatures from 1000 to 1400degreesC for 100 h, in relationship with the microstructure of the exposed surfaces. The material starts to oxidize notably at temperatures higher than 1300degreesC. The oxidation kinetics is parabolic in the temperature range 1350-1450degreesC, the oxidation products are dependent on temperature and exposure time and are mainly constituted by crystalline mullite and alumina. The surface modification induced by long term oxidation does not affect mechanical strength until 1200degreesC, while after oxidation at 1400degreesC, the residual strength is about 25% of the starting one. These results are discussed in terms of the microstructure modifications induced by oxidation. (C) 2004 Kluwer Academic Publishers.