In this study, the oxidation behaviours of Y, Sin and Y/Sm doped alpha-SiAlON ceramics are investigated to examine the oxidation kinetics. The oxidation tests are carried out at 1300-1450 degrees C for different holding times (6-72 hours). Measurements of weight gain and observations of oxidized surfaces were made. The results show that up to 1400 degrees C, oxidation resistance of Sin-doped samples is better than dual cations (Y/Sm) doped compositions even though the former has the higher amount of grain boundary phase. Results are discussed based on grain boundary phase, oxidation morphology and thickness.