Silicon nitride, sintered with the aid of alumina and yttria, was exposed at 1000 degrees C to two different simulated gas turbine environments. The composition of the reaction gas was varied by delivering either a high or low sulphur fuel into a burner rig. The characteristics of the corrosion product varied markedly with the sulphur content of the fuel. The extent of silicon nitride degradation was examined by two techniques: weight change and 4-point flexural strength. Strength measurements were conducted both at room temperature and at 1000 degrees C. Whereas the weight gain information revealed that corrosion was enhanced in the low sulphur combustion gas, the strength of the corroded silicon nitride did not vary significantly from that of the as-received material. Scanning electron microscopy of the fracture surfaces was utilised to identify the fracture origins in the as-received and corroded samples. Strength, even after corrosion, was controlled primarily by defects introduced during manufacture.