Cobalt-iron transition-metal disilicides were in investigated by Mossbauer effect and x-ray diffraction in order to determine the concentration range of their homogeneous and separate phase formation. Except at low Co or Fe concentrations, Co and Fe formed separate CoSi2 and FeSi2 phases. Up to 10 at % Co was found soluble in beta-FeSi2; Fe dissolved in CoSi2 below 1.5 at % and was positioned at two different sites: of cubic symmetry. The results obtained for the phase formation in thin layers of epitaxial CoSi2 on Si implanted with Fe were in agreement with the results obtained fur the bulk samples. (C) 1998 American Institute of Physics.