It has been concluded that the preliminary dry grinding Of alpha-Si3N4 and beta-Si3N4 in a planetary ball mill dramatically increases their reactivity towards the corrosion by oxygen and water at high temperature (1480degreesC). It has been shown that the reactivity of both phases with water vapour is considerably higher than that with oxygen. Moreover, alpha-Si3N4 has a corrosion resistance considerably higher than beta-Si3N4 in spite of the fact that alpha-Si3N4 ground samples have a smaller crystal size and a higher concentration of microstrains than the ground beta-Si3N4 samples. The different corrosion resistance of both silicon nitrides has been attributed to the difference in their crystalline structures, what would lead to different oxidation mechanisms. Amorphous silica is the product obtained from the oxidation of alpha-Si3N4 by oxygen or water, while alpha-cristobalite is the oxidation product obtained from the corrosion of beta-Si3N4. (C) 2004 Kluwer Academic Publishers.