The rates of oxidation of yttria hot‐pressed silicon nitride were measured at temperatures from 1620° to 1796°K in 20 kPa of oxygen for times to about 25 hr. Measurements were made by volumetric and gravimetric methods. The oxidations were parabolic in the range studied, and the rate constants were much smaller than those for magnesia hot‐pressed silicon nitride. Two regimes of activation energy were observed with values of 450 kJ/mole at low temperatures and 980 kJ/mole at high temperatures. Examinations of the corrosion layers by electron microscopic techniques showed that beneath the oxide layer formed there was an equal thickness of corrosion‐affected material in which yttrium was depleted.