The experimental results of this study of the thermal oxidation of GaAs under atmospheric conditions and in dry oxygen over a temperature range of 400–530‡C are presented. It is shown that for temperatures in the range of 400–450‡C the oxide layers are formed according to a parabolic law while for temperatures in the range of 480–530‡C they are formed according to a linear law. An interesting dependence of the reflectivity of GaAs with an oxide layer on the thickness of this layer during the initial stage of the oxidation process is described, giving evidence of the complex structure of the layer formed.