High-temperature oxidation behavior, microstructural evolution, and oxidation kinetics of additive-free alpha-, beta-, and mixed silicon nitride ceramics is investigated. The oxidation rate of the ceramics depends on the allotropic ratio; best oxidation resistance is achieved for ceramics rich in a-phase. Variations in the oxidation kinetics are directly related to average grain size and glass distribution in the oxidation scale. The oxygen contents incorporated into the Si3N4 phase before its dissolution at the oxidation front affects the local glass composition and thereby yields nucleation and growth rates of SiO2, crystallites within the glass phase and a final oxidation scale microstructure, which depend on the incorporated oxygen contents. For the alpha-polymorph, the dynamic oxygen solubility is found to remain negligible; therefore, a nitrogen-rich glass forms at the oxidation front, which promotes devitrification and yields a scale with small grain size and thin intergranular glass films. beta-(SiN4)-N-3 is observed to form oxygen-rich solid solutions on oxidation, which are in contact with silicon oxynitride or oxygen-rich glass. Nucleation of cristobalite in the latter is sluggish, yielding coarse-grained oxidation scales with thick intergranular glass film.