Diffusion barriers play an important role in numerous phase formation processes. A well known exam- ple in microelectronics is the reactive diffusion growth of silicide thin films which are applied as contact materials. In this work, the effect of a Ti interlayer on the kinetics of the formation of CoSi by reactive diffusion is investigated. Therefore, Co(100nm)/Ti(5nm) deposited on Si(111) is annealed at various tem- peratures. Transmission electron microscopy and atom probe tomography allow to study the evolution of microstructure and local compositions after each annealing. It is observed that the CoSi growth starts at the Ti/Si interface and it is controlled by Co diffusion through the Ti interlayer. The Ti interlayer keeps its microstructure during the growth of CoSi. At higher annealing temperatures, Si diffusion through the Ti interlayer to the Co layer is evidenced. First, it segregates at grain boundaries of the polycrystalline Co layer on top of the Ti interlayer before it reacts to cobalt silicides. Also Ti diffusion to the CoSi/Si in- terface occurs that may affect diffusion processes and phase formation reaction at this interface. Beyond the experimental observations, a model is developed to quantify the diffusivity of Co in the Ti interlayer on the base of the investigated CoSi growth. Furthermore, a model is developed in order to estimate the diffusivity of Si in the Ti interlayer on the base of the formed solid solution and segregation in the Co layer. Both models can be generally applied for similar material configurations to estimate diffusivities in interlayers or to predict phase growth.