The effect of different sintering additives on the high temperature oxidation and corrosion behaviour of Si3N4 ceramics is investigated. A corrosion furnace was set up which allows testing of ceramic discs at test temperatures upto 1500°C. It was possible to conduct tests under ambient oxygen conditions, or with increased water vapour as well as testing with corrosive gases such as HCl or SO2. Si3N4 was prepared with MgO, Al2O3, Y2O3 and Al2O3+Y2O3 sintering additives. These discs were subjected to different oxidation and corrosion test conditions at temperatures of 1200°C and 1500°C for upto 128 h. The effects that the different corrosion enviroments have on the corrosion resistance of the Si3N4 materials are presented.