Highly resolved Co depth profiles have been obtained during the initial stages of Co growth on Si(100) at low temperature (−60 • C) by in situ high-resolution Rutherford backscattering spectrometry. We found extensive Co in-diffusion in the submonolayer growth regime even at this low temperature, besides Co on top of the Si surface. The amount of diffused-in Co is larger than the amount of Co at the Si surface. Every second Si layer is depleted of Co, starting at the Si surface, thus giving rise to compositional oscillations of Co in the Si(100) lattice. At this low temperature the growth of metallic Co on the Si surface is observed at 0.1 ML of deposited Co, which continues for higher coverages. At much higher coverage (5.93 ML of Co) almost exclusively low Co content silicides are formed at the Co/Si interface. The data presented here are compared with previous room temperature deposition data and are different in several aspects.PACS 68.55.ag; 82.80.Yc