Thin films of tin were deposited on niobium. tantalum and molybdenum substrates. The energy spectrum of the reflected electrons was recorded to determine the energy loss mechanisms. The energy loss spectra of the thin films differed from those of the pure metals. Peaks were observed in the loss spectrum of the thin films which could be attributed to the excitation of interface plasmons at the boundary between the tin film and the substrate. The energy of the interface plasmon is a few percent less than the theoretical value calculated for a nearly free electron model.