Co-Si surface layers were prepared by implanting radioactive 57Co into an evaporated Co layer on top of a Si single crystal. This system was subsequently thermally annealed and the formation of the different cobalt silicides was followed as a function of anneal temperature, by Mössbauer spectroscopy. Rutherford backscattering spectroscopy measurements show the formation of parallel silicide layers on top of the Si substrate. The final silicide formed, after annealing above 600° C, is CoSi 2. Buried silicide layers were obtained by implanting 160 keV 59Co (alternated with small fractions of 57Co) up to a dose of 2 × 10 17 at./cm 2. After thermal annealing above 800°C, the spectrum of CoSi 2 appeared. All the CoSi 2 layers show an anomalous Mössbauer spectrum: although CoSi 2 is known to be cubic, and therefore is expected to give rise to a single line Mössbauer resonance, the spectrum always contains a broad side resonance, indicating a non-cubic surrounding for a large fraction of the Co-atoms.