The oxidation process of Si2N2O, prepared by a hot isostatic pressing technique, has been studied by the thermogravimetric method. The oxidation has been performed in oxygen for 20 h in the temperature range 1300° to 1600°C, producing oxide scales of amorphous SiO2and α‐cristobalite. The weight gain forT1350°C does not begin to follow a parabolic rate law, until a certain time,t0. TheA0parameter in the parabolic rate law, (Δw/A0)2=Kpt+B, represents the cross section area,A, through which the oxygen diffuses; in the derivation of this lawAis assumed to be constant during the experiment. If crystallization occurs during the oxidation process,Awill decrease with time. A function,A(t), describing the time dependence, has been developed and incorporated into the parabolic rate law, yielding a new rate law, which reads ΔW/A0=aarctan √bt+c√t. This new rate law is valid in the time intervaltt0, the oxidation process follows the equation (Δw/A0)2=K°pt+B0. The relation of the latter equation to the common parabolic rate law is described. All of the oxidation curves are described by these equations. The activation energy of the oxygen diffusion (and of the oxidation (Kp)) is found to be 245 ± 25 kJ/mol, which is consistent with literature values reported for ox