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 degrees C), amorphous phase developed on Si(100). At higher temperatures epitaxial hexagonal GdSi1.7 was found on Si(111), while on Si(100), epitaxial orthorhombic GdSi2 was formed. For thicker gadolinium films on Si(111), a conventional diffusion-reaction process appeared. The hexagonal GdSi1.7 phase formed first and then transformed to the second phase (orthorhombic GdSi2). The ratio of these phases could be described by a model. On Si(100) substrate at each thickness and annealing, only orthorhombic GdSi2 phase was formed. The phase formation depended on the time and temperature of the annealing and even on the initial Gd Nm thickness and substrate orientation.