Interactions in the Co/Si thin-film system were investigated by MeV backscattering and x-ray-diffraction techniques. It was found that Si diffuses through the Co layer and accumulates at the sample surface at about 300 °C. Increasing the temperature causes the growth of the Co2Si phase, followed by the simultaneous growth of the CoSi phase. The growth rates for both phases have a square root of time dependence. The activation energies of growth are 1.5 and 1.9 eV for the Co2Si and CoSi phase, respectively. A model for the growth of multiple phases is suggested. The transformation between CoSi2 and CoSi is found to be a reversible reaction.