Thin films of aluminum were deposited on a niobium substrate. The energy spectrum of 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 aluminum film and the niobium substrate. The energy of the interface plasmon is a few percent less than the theoretical value.