4He+backscattering spectrometry and x‐ray diffractometry have been used to study the formation of Ni and Pt silicides in Si(xtl)/Mfilm structures with different metal film thicknesses ranging from 800 to 5000 A˚. Results clearly show that first the phase richer in metal (Ni2Si, Pt2Si) grows and continues to grow until all available metal is reacted, then the phase richer in Si (PtSi, NiSi) starts to grow. Present results prove that in Si(xtl)/Pt or Ni interactions the growth phase is determined by the reaction kinetics and that diffusion of metal atoms through Pt2Si and Ni2Si is fundamental in keeping the reaction going.