Al2O3/Si(001) interfaces were investigated using scanning reflection electron microscopy and x-ray photoelectron spectroscopy. A uniform and stoichiometric ultrathin Al2O3 film of ∼0.6 nm was grown on an atomically flat Si(001)-2×1 surface, and the resulting Al2O3/Si(001) interface was atomically abrupt. Furthermore, an intentional high-pressure oxidation shows that we can grow Si oxide at the Al2O3/Si(001) interface with atomic-scale uniformity as this oxidation proceeds in a layer-by-layer manner. The resulting Si oxide/Si(001) interface was also atomically abrupt. In addition, the rate of oxidation of Si at the Al2O3/Si(001) interface depends strongly on the O2 pressure. © 2001 American Institute of Physics.