Impurities in raw Si3N4 powders remain in intergranular glassy phases in Si3N4 and Si2N2O ceramics and degrade their high-temperature properties. Fluorine is one of the typical impurities in the raw powders. The oxidation rate of Si2N2O ceramics doped with Gd2O3 greatly varied with a difference in impurity contents (especially F) of the raw Si3N4 powders used. When a high concentration of impurity existed in the intergranular glassy phase, the rate of oxidation was controlled by O-2- diffusion through the glassy phase in the partly oxidized scale and unoxidized body; outward diffusion of Gd-3+ occurred concurrently. On the other hand, when the impurity contents in the intergranular glassy phase was very low, the diffusion rate of ions (Gd-3+, O-2-, etc.) in the glassy phase became very low (substantially zero in the oxidation at 1300 degreesC). Only cristobalite (SiO2) was formed on the surface. The rate of oxidation was controlled by O-2 diffusion through the cristobalite layer, and was very low.