Al&sngbnd;SiO&sngbnd;Al&sngbnd;SiO&sngbnd;Au triodes with SiO thicknesses between 150 and 500 Å have been used to measure the potential distribution in thin oxide films before, during, and after the development of voltage‐controlled negative resistance (VCNR) in the current‐voltage characteristics. Development of VCNR in the triode is accompanied by the establishment of a high‐field region about 120 Å in thickness near the negative electrode. If triode potentials are reversed after developing conductivity, VCNR is still found in the current‐voltage (I‐V) characteristic of the triode but the potential distribution in the triode is only slightly changed. VCNR in theI‐Vcharacteristic is a high‐field phenomenon but it does not depend on field emission of electrons from the metal electrodes. Conductivity in the bulk of the insulator is Ohmic with electron mobilities ∼10−3−10−2cm2/V‐sec. The behavior of Al&sngbnd;SiO&sngbnd;Au diodes is identical to that of triodes. Electroluminescence of Al&sngbnd;SiO&sngbnd;Au diodes, which appears when conductivity is developed, is characterized by a steep rise in intensity at 1.8 V, the voltage at which electron emission into vacuum from such diodes is first detected. Both electroluminescence and electron emission provide evidence for high‐energy processes in the oxide film. A phenomenological model of conductivity and voltage‐controlled negative resistance in thin oxide films is developed in which impurity conduction is the most important conduction mechanism.