The initial stages of the reaction of Co withSi0.79Ge0.21(100)were studiedin situwith extended x-ray absorption fine structure spectroscopy and reflection high energy electron diffraction. The Si:Ge ratio in the first coordination shell of Co in sub-monolayer Co films was found to increase with film thickness and annealing temperature, indicating preferential formation of Co–Si bonds. The impact of the observed preference for Co–Si bonding on the morphology of epitaxialCoSi2/Si1−xGexheterostructures is discussed. ©1997 American Institute of Physics.