Metal‐oxide‐semiconductor (MOS) capacitors were prepared by thermal oxidation (wet and dry) of AlAs epilayers grown on GaAs using molecular beam epitaxy (MBE). This oxidation process was found to be self‐terminated at the AlAs‐GaAs interface. Together with the use of MBE for growing the AlAs‐GaAs multilayer structures, the oxide films obtained were extremely uniform over areas ≳6 cm2, possessed sharp oxide‐semiconductor interfaces, and precise film thicknesses. Capacitance‐voltage measurements on these MOS capacitors displayed inversion behavior and showed little or no hysteresis for oxides prepared by dry oxidation. Measured fixed interface charge density was ∼ (4–6) ×1011cm−2, while breakdown fields were ∼ (2–4) ×106V/cm.