Silicon oxynitride ceramics free from sintering aids were oxidized in oxygen-argon and oxygen-nitrogen gas mixtures with varying oxygen and nitrogen partial pressures in the range of 1000 to 1300 degrees C. Oxidation kinetics of Si2N2O followed a parabolic rate law with activation energies ranging from 43 to 52 kcal/mol. The parabolic rate constants for Si2N2O exhibited a linear dependence on the oxygen partial pressure but no dependence on the nitrogen partial pressure. These findings, in conjunction with our established knowledge of the oxide and interface characteristics of oxidized samples, suggested that molecular oxygen diffusion through SiO2 plays a predominant role in the rate-limiting process for the oxidation of Si2N2O.