The backscattered electrons are detected on the energy range 100 eV to 2000 eV. A performant Monte-Carlo program is used to support the interpretation of the results obtained with a retarding field analyser, from Ag and Au samples. Computer simulation is in good agreement with the experiment. Depth penetration study by the computer calculation indicates that, only the two first layers contribute for the main in the backscattering process. In EPES spectroscopy, the fundamental parameter is the percentage η e of the elastically reflected electrons. The objective of this study was to differenciate three kinds of Au - Ag structures. That has been put in evidence by measurements of η e at different primary energies Ep. We have also investigated a sharp interface Ag(200 A°)/Au(500 A°), using a sputtering argon ion gun and a CMA apparatus. The position of the interface is well marked with a precision of two monolayers, while the Auger profile of the same sample shows eight monolayers as interface. As a matter of fact, EPES is a high sensitive method of surface analysis.