Porous Nicalon-alumina compacts with a bulk density of 2.32 Mg/m(3) were oxidized in Ar-25%O-2 gas mixture at 1373-1773 K, and subsequently they were exposed to argon at 1773 K. The mass change of compacts was thermogravimetrically determined during oxidation and exposure. The oxidized and exposed fibers were characterized through AES and XRD analysis. As the oxidation temperature increased, the oxide film was successively transformed into amorphous silica, cristobalite and mullite. The fiber core was only slightly decomposed during oxidation above 1473 K. The oxidation rate could be described by the two-dimensional disc-contracting formula: reaction control at earliest stage and diffusion control at later stage. The oxidation of Nicalon was greatly accelerated by the presence of alumina. Excess thickening in oxide film caused the thermal decomposition of the fiber core during exposure in argon: >0.8 mum for cristobalite film and >1.5 mum for mullite film.