Incoherent processes of multiple electron inelastic scattering by a solid surface are considered, taking into account the effects of bulk and surface excitations. The reflected electron energy loss (REEL) spectra were obtained for silicon and some 3d metals. It was shown that middle-energy electron scattering by a solid surface can be described on the basis of a bilayer model with good accuracy. The approach suggested allows determining the differential probability of electron inelastic scattering (DPEIS) both on bulk and surface excitations from the experimental REEL spectra.