The oxidation behavior of β-SiAlONs (Si6-zAlzOzN8-z, z = 1, 3, and 4) was investigated at temperature ranging from 1473 to 1673 K in a (N2-3%O2)-20%H2O atmosphere. Oxidation kinetics was followed on the basis of the mass gains, and the oxidized specimens were characterized by FE-SEM, XRD, and EPMA. The mass gain was found to clearly increase with temperature and the z value. For oxidation at 1473 and 1573 K, the gain in mass was observed to be small. During oxidation at 1573 K, mass loss occurred. This loss may be because of the formation of volatile SiO(OH)2. The changes in mass depended on the z value. On the other hand, the oxidation rates at 1673 K were found to be higher than those at and below 1573 K by more than one order of magnitude. The oxidation kinetics at 1673 K followed an almost linear rate law. The XRD and EPMA results showed that the oxide formed on β-SiAlON (z = 1) was composed of a mixture of amorphous aluminosilicate and mullite, whereas that on β-SiAlONs (z = 3 and 4) was composed only of mullite.