Thermal oxide films formed on n-GaAs(100) and AlxGa1-xAs (x = 0.25-0.80) from 450-500degreesC have been characterized by Auger electron spectroscopy and electron microscopy, and the relative oxidation rates of AlGaAs in GaAs-based heterostructures and InAlAs in InP-based heterostructures have been determined. The kinetics and mechanism of oxidation depend on the particular oxidant. Selective oxidation of Al-containing layers in device structures depends on the layer thickness but is independent of the dopant level, and is optimized by oxidation in moist nitrogen (95degreesC). Electrical measurements have been performed on oxidized InAlAs/InGaAs heterostructure diodes.