Wet oxidation of alpha -Si3N4 and beta -sialon (Si6-zAlzOzN8-z, z = 1, 2, and 3) powders was carried out at 1000-1300 degreesC in atmospheres containing 0-20 kPa water vapor. The oxidation was monitored by XRD, Si-29 and O-17 magic-angle spinning-nuclear magnetic resonance (MAS-NMR), transmission electron microscopy, and thermogravimetric analysis. The kinetic results were compared with those of our previous dry oxidation studies in Ar/O-2(80/20 kPa). The oxide product phase was composed of very fine acicular mullite grains and amorphous SiO2. The early-stage oxidation is described by a two-stage linear kinetic law, the first stage operating at 0-5% reaction, the second at 5-20% reaction. From 20-80% reaction, the kinetics follow a parabolic law represented by the Ginstling-Brounshtein equation. The diffusion rates for wet oxidation were 2-11 times greater than for dry oxidation. The dependence of the diffusion rate on water vapor pressure over the range 0-20 kPa was approximately parabolic, indicating the diffusion of OH-. When the z = 3 sialon was oxidized in either Ar/(H2O)-O-17 or Ar/O-2/(H2O)-O-17 atmospheres, O-17 was detected by MAS-NMR in the SiO2 and mullite oxidation products, indicating that the oxygen atoms from the water vapor contribute in nearly equal amounts to the formation of mullite and SiO2. (C) 2001 The Electrochemical Society. All rights reserved.