There is experimental evidence of the existence of a quite thick non-stoichiometric Si-SiO2 transition layer in MOS structures but, the observation of oscillations in the Fowler-Nordheim (FN) current is an experimental proof of a very abrupt chemical change at the Si-SiO2 interface. In this paper it is shown that these two facts are compatible if one considers that the non-stoichiometric layer is due to roughness at an atomic scale. The concentration of the FN current in a small fraction of the total oxide area has been found to prevent the randomization of the phase of the wave functions of the ballistic electrons. The effects of the interface roughness on the oscillations have been studied and a new technique for roughness characterization has been proposed. As a main conclusion, we have demonstrated that the Si-SiO2 interface has to be considered simultaneously rough and chemically abrupt. © 1989.