An S-type negative-resistance region in the current/voltage characteristics of thin films of silicon oxide and aluminium oxide is reported. Radio-frequency current oscillations associated with this region were observed. Related to this S-type negative resistance was a switching phenomenon, whereby the film could be switched from an insulating to a conducting state, and vice versa, by the application of suitable pulses. This could be done repetitively in most samples without any change in their dielectric properties.