The reactions during annealing of a Co/Ti bilayer structure on Si(001) were studiedinsituto reveal the roles of the Ti interlayer and the annealing ambient on the formation of epitaxial CoSi2. We shown that an oxygen contaminant in the nitrogen annealing gas is needed to form a stable, Co–Ti–O (spinel) membrane at the metal/Si interface that limits diffusion and is crucial for the perfection of epitaxial CoSi2. Annealing in vacuum or otherwise inert environments led to polycrystalline CoSi2films and no spinel phase. ©1995 American Institute of Physics.