For Co in Si, the competition between CoSi2 formation and cavity trapping is studied by Rutherford backscattering and Mossbauer spectroscopy. The presence of nanosized voids hampers the formation of a buried epitaxial silicide layer in its initial phase, preventing the small CoSi2 particles from forming a bulk layer. The Mossbauer spectra show that a pre-existing silicide phase can be partially dissolved in favor of cavity trapping. In addition, channeling measurements provide qualitative information about the voids, showing that the thermal stability of the voids is much higher than for defects resulting from self-implantation. (C) 1996 American Institute of Physics.