The formation of suicides via the thin-film deposition of transition metals onto Si substrates has been investigated by a monoenergetic positron beam. The Doppler broadened spectra of annihilation radiations in the as-deposited state and the silicide-formed state of the Ti/Si and Ni/Si systems were measured as a function of the incident positron energy between 0.1 keV and 30 keV. From the depth dependence of positron annihilation characters, the diffusion length of positron was determined on the basis of one dimensional diffusion models. In the case of the Ti/Si, the formation of the suicides did not induce the shortening of positron diffusion length in Si substrate. On the contrary, the diffusion length of positrons in formed suicides was very short indicating the presence of a great number of positron traps in suicides. From this fact, the moving species during the silicidation may be Ti atoms. On the other hand, in the case of Ni/Si, the silicidation induced a very short diffusion length of positrons in the Si substrate. On the contrary, the diffusion length in the formed suicide is long to a comparable extent expected defect-free metals. From this fact, the moving species may be Si atoms. In the present parer, the first report on the microscopic observation of Kirkendall effect in metal/Si systems by means of a monoenergetic positron beam will be given.