The Au/GaAs interface is currently the subject of much investigation. However, a complete understanding of this interface, and the ability to precisely control Au/GaAs interface properties, is still lacking. Previous work has focused primarily on interfaces prepared on the GaAs(110) cleaved surface and on the chemically or thermally prepared GaAs(100) surface. This paper presents the first Schottky barrier results for the Au/GaAs(100) interface prepared completelyinsituon GaAs grown by molecular‐beam epitaxy. The resulting interface displays unexpected properties which can be interpreted in terms of enhanced electrode interdiffusion. In addition, the capability of molecular‐beam epitaxy forinsituprocessing enables the stabilization of this interface against diffusion, and allows the formation of a Au/GaAs system with nearly ideal properties. Newly developed ballistic electron spectroscopy and imaging techniques demonstrate that the heterogeneity present at the interface of Au/GaAs(100) fabricated on chemically treated GaAs substrates is removed.