The kinetic study of GaAs thermal oxidation in the presence of Sb ₂O₃ + Ga₂O₃ compositions showed a significant negative departure of the oxide film thickness from the additive line for low Ga₂O₃ concentrations. When the Ga ₂O₃ concentration in the compositions was higher than 40 mol %, the oxide film thickness grown on GaAs was an additive function of the activator composition. The nonlinear effects observed were caused by the influence of small Ga₂O₃ additions on the vaporization dynamics of antimony oxide through the enhancement of Sb₂O ₃ sintering during the experiment.