When thin overlayers on a bulk material have to be analysed by EPMA (electron probe microanalysis) or AES (Auger electron spectroscopy), the signal obtained is enhanced by three factors: (i) back-scattering, (ii) characteristic x-rays induced in the bulk by the primary beam and (iii) continuous x-rays (bremsstrahlung) created also in the bulk. The Monte Carlo techniques are applied in order to evaluate the above three enhancement factors when the primary energy is varied in the range 5-50 keV. It is shown that at high primary energies the second and third factors are no longer negligible and become significant in some cases. The approximation to ignore the third factor in EPMA and both the second and third factors in AES is not always valid and introduces an appreciable error. Results are shown for the case of thin films (Al, Au, Cu) on a bulk material (Au, Cu, Si, Ni, Pt).