Diffraction anomalous fine structure on both Fe and Co K edges and transmission electron microscopy are used to study the crystallographic structure and morphology of thin ternary silicides films grown on Si(1 1 1). Co1-x,FexSi2 layers (about 10 nm thick) have been grown by UHV-MBE in epitaxy on silicon. On either oriented or slightly mis-oriented (1 1 1) surfaces, and in situ annealed at about 925 K. In the Fe-rich part of composition, annealing at 925 K results in a phase separation into grains of two different structures, namely a CaF2-type (as CoSi2) and alpha-FeSi2-type Structures. In contrast, for Si surfaces with a very low miscut and in the Co-rich part of composition. annealing at 925 K results in an almost single phase with a CaF2-type structure. Transmission electron microscopy images show that, even in the Co-rich past, the stress gradient generated at the step edges acts as a nucleation centre for the alpha-FeSi2 phase, which is thus probably more stable than the CaF2-type phase, stabilised by the stress on the terraces. (C) 2002 Elsevier Science B.V. All rights reserved.