Insulating films consisting of both aluminum oxide (Al2O3) and native oxide (Ga‐As‐O) have been grown anodically onp‐ andn‐type GaAs in a dc oxygen plasma. Films were grown at a constant anodization current up to 2000 A˚ in thickness using GaAs wafers precoated with 50 to 150 A˚ of Al. Auger analysis using ion milling for depth profiling revealed Al2O3mixed with native oxide in the upper 1000 A˚ and only native oxide at the GaAs interface. It was found that the presence of the Al2O3did not prevent the buildup of As at the GaAs interface during anodization. The composite films were found to be ohmic with a resistivity of 1015&OHgr;‐cm for electric fields below about 6×105V/cm. The presence of the Al2O3increased the breakdown strength to about 4×106V/cm. An analysis of the capacitance‐voltage characteristics of metal‐oxide‐GaAs devices measured from 100 to 1 MHz showed a typical interface‐state density of 2–6×1012cm−2 eV−1in the lower half of the GaAs band gap. In contrast to earlier reports, the addition of Al2O3to the anodic oxide did not alter the As and Ga distributions in the native oxide adjacent to the GaAs substrate nor did it appreciably improve the electrical properties of the oxide‐GaAs interface.