As/Sb and Sb/As anion exchange reactions are characterized and modeled in order to better understand and control mixed-anion heterojunction synthesis by molecular-beam epitaxy. The importance of substrate temperature, anion flux exposure time, and incident anion molecular species is examined. Sb/As structures exhibit Sb surface segregation, and the subsequent incorporation of the segregating surface population into epitaxial overlayers. As/Sb exchange is significant in extent of the reaction, and enhanced with incident dimer fluxes. Competitive reactions between exchange and isoelectronic compound formation yield complex structures. (C) 2004 American Vacuum Society.