18O was implanted in the Si substrate or in the Co overlayer to study the effect of oxygen and its redistribution during Co silicides formation. Implanted and unimplanted samples were annealed in vacuum at 450°C for different durations. Over the investigated dose range of 0.5 to 2 × 1016 18O/cm2 in the Co film, the phases formed upon annealing are Co2Si and CoSi, as for the unimplanted samples. The kinetics of the silicide growth is changed little by the presence of oxygen in the Co film. Oxygen accumulates at the Co2Si/Co interface. When the oxygen is implanted in the Si substrate, the higher the oxygen dose is, the slower (fasteil the growth rate of CoSi(Co2Si) becomes. Below 1 × 1016 O/cm, both phases (i.e. CoSi and Co2Si) still form; above 2 × 1016 O/cm2 , only Co2Si formsWhen two phases form, the oxygen moves deep into the Si substrate; when only Co2Si forms, the oxygen moves toward the surface.