The formation of amorphous and equilibrium phases was investigated during the solid-phase reaction of Gd thin film with (111) and (100) oriented Si substrate as a function of thickness and annealing by X-ray diffraction, Rutherford backscattering and transmission electron microscopy. For Gd films thinner than 30 nm, the phase formation was affected by the substrate orientation. At low temperature (320°C), amorphous phase developed on Si(100). At higher temperatures epitaxial hexagonal GdSi 1.7 was found on Si(111), while on Si(100), epitaxial orthorhombic GdSi 2 was formed. For thicker gadolinium films on Si(111), a conventional diffusion–reaction process appeared. The hexagonal GdSi 1.7 phase formed first and then transformed to the second phase (orthorhombic GdSi 2). The ratio of these phases could be described by a model. On Si(100) substrate at each thickness and annealing, only orthorhombic GdSi 2 phase was formed. The phase formation depended on the time and temperature of the annealing and even on the initial Gd film thickness and substrate orientation.