Backscattered electrons are more efficient at producing secondary rather than primary electrons. Using a thin layer of oxidized aluminium on five different substrates (C, Al, Cu, In and Au), we have measured how the efficiency ratio ( beta ) varies as a function of primary electron energy, thereby characterizing quantitatively this well-known effect. We also carried out Monte Carlo simulations to calculate beta for thin Al films on Be, Cu and Au substrates. Interestingly, these metallic films behave in a similar manner to the insulating films investigated experimentally.