The applicability of in situ X-ray diffraction for studying reactions characterizing thin silicide film formation was explored. The Co-Si system was chosen as a test case for the validity of the technique, because the importance of this system in very-large-scale integration microelectronic applications. It is shown that the technique is useful for kinetics and phase relations studies. CoSi is the only phase detected by in situ X-ray diffraction in specimens annealed in the range 400-450 degrees C. Clear coexistence of CoSi and CoSi2 is indicated in the samples annealed at 500-550 degrees C. Above 600 degrees C in situ X-ray diffraction revealed that only CoSi2 exists. An activation energy of 1.2 eV was derived from the in situ diffraction data for the diffusion-controlled growth of CoSi2. It is assumed that this value is associated with grain boundary diffusion.