The microstructure and interface properties of AlxGa1- xAs materials that have been laterally oxidized in wet N 2 for several compositions (x equals 0.80, 0.82 . . . 1.00) and temperatures (360 degrees Celsius to 450 degrees Celsius) have been studied. The micro-structure is found to be relatively insensitive to composition and oxidation temperature. The oxidation forms an amorphous solid solution (AlxGa1-x)2O3 that transforms to polycrystalline, γ-(AlxGa1-x)2O3 under electron beam exposure in the electron microscope. Evidence suggests a small fraction of crystalline (AlxGa1- x)2O3 is formed via post oxidation annealing of the oxide. The level of hydrogen present in the oxidized layers is 1.1 multiplied by 1021 cm-3, which is too low for the amorphous phase observed to be a hydroxide rather than an oxide. The amount of As in the layer is reduced to less than 2 atm%, and no As precipitates are observed. The (AlxGa1-x)2O3/GaAs interface is abrupt, but prolonged oxidation will cause the GaAs to oxidize at the internal interfaces. The reaction front between the oxidized and the unoxidized AlxGa1-xAs has a 10 to 20 nm-wide amorphous zone that shows a different contrast than the remainder of the amorphous oxide and is stable under electron irradiation.