It has been demonstrated, using synchrotron radiation, that at rapid thermal annealing rates (3°C/s) the formation of CoSi2 shifts to higher temperatures when a thin Ti interlayer is placed between Co and polycrystalline Si. It has also been shown that the Ti interlayer reduces the temperature range between the start of CoSi formation and CoSi2 formation (i.e. the range over which CoSi is present). 13 nm of Co deposited by physical vapor deposition on polycrystalline Si with and without either a 2 nm or 3.4 nm interlayer of Ti was analyzed in-situ by monitoring x-ray diffraction (XRD) peak intensity as a function of temperature using monochromatic radiation from a synchrotron beam line and by monitoring resistivity as a function of temperature in a rapid thermal annealing (RTA) system. The XRD analysis indicates that the phase formation proceeds from CoSi to CoSi2 in a temperature range that decreases from about 200°C to 140°C to 115°C with pure Co, Co/2 nm Ti and Co/3.4 nm Ti films respectively. The onset of the CoSi formation increases by about 135°C and 160° for Co/ 2 nm Ti and Co/3.4 nm Ti compared to pure Co. The CoSi temperature range decreases from about 75°C in pure Co to less than 50°C in Co/Ti. In-situ RTA resistance along with in-situ XRD analysis indicates that the onset formation temperatures for CoSi are about 440°C, 575°C and 600°C and the temperatures for the completion of CoSi2 formation are about 640°C, 715°C and 715°C for Co, Co / 2 nm Ti and Co / 3.4 nm Ti films respectively. The results are consistent with the Ti interlayer acting as a diffusion barrier during the initial stages of the Co-Si reaction.