The formation of Ni3Si2 from the reaction of Ni 2Si with NiSi, and that of Pt6Si5 from the reaction of Pt2Si with PtSi have been investigated by Rutherford backscattering, x-ray diffraction, resistance measurements, and optical and electronic microscopy. Standard x-ray diffraction patterns were calculated for Pt6Si5 and for the high-temperature form (hexagonal) of Pt2Si. These are shown to match experimental diffraction patterns. Both Ni3Si2 and Pt6Si5 form quite suddenly (at 470 and 535°C, respectively) according to the pattern of nucleation-controlled reactions which are anticipated when the free energies of formation of the new phases are sufficiently small. These observations are discussed with respect to the absence of both Ni3Si2 and Pt6Si5 from the sequence of phases which form when Ni and Pt thin films react with Si. Resistivity measurements are reported for Ni 3Si2, Pt6Si5, and for the two forms (low and high temperature) of Pt2Si.