The oxidation behaviour of a low-cost porous Si2N2O-ZrO2 composite material, produced without the use of a sintering aid, was investigated in dry and humid air and argon, respectively, in the temperature range 800-1470 degrees C. The extent of internal oxidation in dry air was found to be dependent on the oxidation temperature. The material exhibits excellent oxidation resistance at temperatures greater than or equal to 1300 degrees C. A very thin layer containing zirconium silicate which is formed on the internal pore walls, appears to protect the material against further oxidation. External pore sealing was not observed. Water vapour was found to enhance oxidation significantly. It was also found that a short-term oxidation treatment at high temperature protects the material from further oxidation at lower temperatures. External pore closure did not occur.