Recent studies of plasma‐enhanced chemical vapor deposition of SiO2upon Si epitaxial layers growninsituupon epitaxial GaAs layers, with the deposited interfacial Si of order 10 Å, strongly suggest that greatly reduced state densities at SiO2/GaAs interfaces have been attained with this approach. We report spectroscopic ellipsometry results for molecular‐beam epitaxy (MBE) Si/GaAs samples both before and after SiO2deposition, as well as before and after annealing. The ellipsometric results indicate that the SiO2/GaAs interface is<9 Å thick. The amount of unreacted silicon ranged from 3±3 to 0.4±0.2 Å equivalent thickness for the best fit cases. Ellipsometric results for samples without the deposited SiO2suggest of order 0.4‐Å unoxidized silicon at the interface. Results of x‐ray photoelectron spectroscopy (XPS) studies of the samples (without deposited SiO2) suggest that all silicon is being oxidized prior to SiO2deposition for at least some of the ‘‘unpinned’’ samples. Raman studies find no Si–Si bonds for some of the unpinned samples. Our data strongly suggest that the reduced interface state density is not dependent upon the presence of unoxidized silicon, at least in the amount required to form the Si/GaAs heterojunction previously assumed in explaining this improved interface. We further report preliminary electrical data for a sample prepared with no silicon deposition; indentical heat treatments on this sample gave similar results, i.e., reduced interface states forn‐GaAs metal–oxide semiconductor capacitors.