Interfacial reactions of ultrahigh vacuum deposited yttrium thin films on atomically clean (111)Si at low temperatures have been studied by both conventional and high‐resolution transmission electron microscopy, Auger electron spectroscopy, and x‐ray diffraction. A 10‐nm‐thick yttrium thin film, deposited onto (111)Si at room temperature, was found to completely intermix with Si to form an 11‐nm‐thick amorphous interlayer. Crystalline Y5Si3and Si were observed to nucleate first within the amorphous interlayer in samples annealed at temperatures lower than 200 °C. Epitaxial YSi2−xwas found to be the only phase formed at the interface of amorphous interlayer and crystalline Si in samples annealed at temperatures higher than 250 °C. In as deposited 20‐ to 60‐nm‐thick Y thin films on silicon samples, crystalline Y5Si3, Si, and YSi and a 2.5‐nm‐thick amorphous layer were found to be present simultaneously. Good correlations were found among difference in atomic size between metal and Si atoms, the calculated free energy of mixing as well as the critical and maximum amorphous interlayer thickness for the Y/Si and a number of refractory metal/Si systems. The Y/Si system is the only system found up to date among all metal/Si systems that theainterlayer can be grown to a thickness exceeding 10 nm during deposition at room temperature.