Nonoxide ceramics of hot-pressed AlN, hot-pressed Si₃N₄, sintered Si₃N₄, hot-pressed sialon and hot-pressed B₄C were oxidized in wet nitrogen and/or wet air atmosphere at 1.5 to 20kPa of water vapor pressure and in dry air at 900-1400°C. All specimens were oxidized by both dry air and water vapor at high temperatures. AlN formed α-Al₂O₃ film on the surface above 1150°C. The oxidation rate of AlN increased with increasing water vapor pressure. The oxidation rate of AlN in wet nitrogen was much greater than that in wet air. Si₃N₄ and sialon formed oxide films consisting of SiO₂, Y₂Si₂O₇ and YAlO₃. The oxidation kinetics obeyed the usual parabolic law. The oxidation rate in wet nitrogen, however, was unaffected by water vapor pressure and increased with increasing Y₂O₃ content. The apparent activation energy for the oxidation of Si₃N₄ based ceramics was about 800kJ/mol. On the other hand, since the oxidation of B4C by water vapor and dry air above 900°C resulted in weight loss, the formation of BO or HBO₂ seemed to be the main reaction. The oxidation rate of B₄C by water vapor above 900°C could be expressed by the surface chemical reaction controlled kinetics with the apparent activation energy of 200kJ/mol.