Theoretical calculations are performed for the electron-energy-loss spectrum, in specular-reflection geometry, of a semi-infinite target made of two polar materials with a sharp interface perpendicular to the free surface of the target. First a general formalism is proposed, which allows for an arbitrary compositional variation of the target along one direction parallel to the surface. As an application, we next perform calculations for a GaAs/AlAs transverse-interface system. It is shown that electrons reflected at short distances (<20–30 nm) from the boundary between the two materials probe optical vibrations located on the interface, as well as phonons that propagate on the free surface of both mediums. The effects of the orientation of the electron plane of incidence, with respect to the interface of the target system, are also investigated.