S-structures with pure SiO 2films. with a nitrided SiSiO 2 interface region or with an insulator layer with uniform weak nitridation throughout the layer were irradiated with electrons with an energy of 2 MeV in dose steps between 10 krad and 2 Mrad (SiO 2). with and without an electrical field between the electrodes. The electrical characterisation using CV (capacitance voltage) and DLTS (deep level transient spectroscopy) measurements gives us information about the charge states in this structure. The electron irradiation generates electron-hole pairs, and a positive gale field causes a hole drift to the interface where we can find a positive oxide charge and a rising interface density of states. The amount of the density of states is a function of the nitridation and of the electron beam dose. The best electrical stability is found under irradiation in a uniform nitrided SiO 2 layer. For this film the fixed oxide charge and the interface state density are not a function of the irradiated dose and not essentially higher than in pure oxide.