We attained a wide modulation of the Si(111)–Au interface reaction by deposition of controlled amounts of Cr on the semiconductor surface beforeinsitujunction formation. Synchrotron radiation photoemission studies show that Cr deposition gives rise to a Si/Cr reacted interlayer that dramatically affects the subsequent Si–Au interdiffusion. Negligible interdiffusion of Cr and Au atoms was found in all cases so that the Si–Au intermixing depends upon an exchange reaction at the Si/Cr–Au interface in which Si is the only moving species. Large reduction of the interdiffusion occurs sharply above a critical value of Cr deposition (10 monolayers) that corresponds to a fully reacted Si(111)–Cr interface.