A switching action has been observed in a two-terminal sandwich structure of the form silicon-silicon-dioxide-aluminium. The application of an increasing positive bias to the aluminium electrode results, at some particular bias, in a change from a high-resistance to a low-resistance state. The reverse change of state is accomplished by the discharge of a condenser across the device. The electrical properties of the device are compared with those of other switching films reported by different workers. It is shown that in the case of silicon dioxide films the switching action does not appear to result from the growth in the oxide of a metallic filament, but rather that it is associated with impurities incorporated in the oxide during growth and device preparation.