The oxidation behavior of three different <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">ZrB</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>ZrB2 -based ceramics, a monolithic <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">ZrB</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>ZrB2 and two <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">ZrB</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>­</mml:mo><mml:mi mathvariant="normal">SiC</mml:mi></mml:math>ZrB2­SiC composites, was studied up to 1350°C and under isothermal conditions at 1120°C for 20 h. Oxidation kinetics and microstructural changes in the oxidized specimens indicate that the monolithic <mml:math display="inline"><mml:msub><mml:mi mathvariant="normal">ZrB</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>ZrB2 has low thermal stability. On the contrary, the introduction of SiC particles markedly improves oxidation resistance due to the formation of an adherent and protective borosilicate glass layer that coats the sample surface, effectively limiting the inward diffusion of oxygen toward the reaction interface. The influence of the secondary grain boundary phases on oxidation is discussed. © 2003 The Electrochemical Society. All rights reserved.