We have investigated the compositional, structural, and band bending effects of growing ultrathin AlAs interlayers between epitaxial NiAl andn‐GaAs(001) by molecular‐beam epitaxy (MBE). Such an interface is of interest because of the possibility of increasing the already large Schottky barrier height at the NiAl/GaAs interface by inserting a lattice‐matched interlayer of larger band gap. We have found that the Fermi energy in the GaAs band gap, as measured by x‐ray photoemission spectroscopy (XPS), is only 0.35–0.40 eV below the conduction‐band minimum when very high quality AlAs is grown onn‐GaAs(001) by MBE. This result is presumably due to the creation of an interface of very high crystallographic quality and the related low density of interface states. We have also measured the valence band offset by XPS to be 0.40±0.07 eV, independent of band bending. Subsequent MBE growth of NiAl at 250 °C invariably increases band bending at the AlAs/GaAs interface, leaving the Fermi energy ∼0.8 eV below the conduction‐band minimum on the GaAs side. This increase in band bending is accompanied by disruption of the AlAs/GaAs interface, which appears to be driven by diffusion of Ni atoms through the thin AlAs interlayer. Post‐growth annealing at 570 °C increases the extent of disruption at the AlAs/GaAs interface and drives the Fermi level to ∼1.0 eV below the conduction‐band minimum. Combining the valence‐band offset and band bending measurements, the barrier height of the NiAl/AlAs/GaAs(001) interfacial system is estimated to be 1.15–1.35 eV, which is nearly a factor of 2 larger than values exhibited by conventional metal/GaAs interfaces.