In the present paper, the interfacial reactions at the interfaces of the Sn/Si system have been studied by means of tunable-sampling-depth electron energy loss spectroscopy (TELS) in conjunction with low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). It is found that, depending on the orientation of the silicon substrates, the interfacial reactions are qualitatively different. No intermixing of Sn with Si occurs at the as-deposited Sn/Si (111) interfaces and the Sn/Si (111) interfaces annealed at temperatures lower than 400℃. However, Sn intermixes with Si at the Sn/Si (111) interfaces annealed at temperatures between 400 adn 700℃, forming a Sn/Si intermixing layer with a maximum thickness of about 1nm. The bulk plasmon peak of this intermixed layer on the ELS curve is at 15. 5eV. No detectable intermixing of Sn with Si has been found at the Sn/Si (001) interfaces annealed at any temperature. Annealing at increasing temperatures only makes the homogeneous Sn layer to become islands, and the islands become smaller and eventually disappear at a temperature higher than 1000℃. A prolonged annealing at 550℃ gives rise to the appearance of (113) facets on the Sn/Si (001) surface, as shown by the 'moving spots' on the LEED pattern.