CO/Si systems were ion beam mixed at 77 K using a 100 keV Ar beam. The formation of different phases as a function of irradiation dose has been studied, using Mossbauer spectroscopy (MS) and Rutherford backscattering spectroscopy (RBS). It was found that Co2Si, CoSi and CoSi2 are formed subsequently in parallel layers. After high dose irradiation , a phase with stoichiometry Co:Si equal to 1:3 was observed, suggesting CoSi3 has been formed. However, MS gave clear evidence that this phase consists of precipitates Of CoSi2 and Si. Finally, we found that the amount of mixing scales linearly with the square root of the fluence, with a mixing rate of 1.0 x 10(4) angstrom 4.