Voltage-controlled negative resistance (VCNR) can be established in metal-insulator-metal structures by applying a potential to the diode, which is usually in vacuum. Light emission due to electroluminescence (E.L.), and electron emission into vacuum, accompany the formation of conductivity; these energetic electronic phenomena are closely related to the "forming" of VCNR and to the resultant current-voltage characteristics. For Al2O3 diodes with impure oxides, VCNR forms at constant voltage, independent of oxide thickness; for clean oxides, forming depends on field and on the metal counterelectrode. The intensity and energy distribution of light emitted from TaTa2O5Au diodes have been measured, and are compared to E.L. from AlAl2O3Au diodes. The voltage threshold for the appearance of E.L. in TaTa2O5Au diodes is 1.2 V, for AlAl2O3Au it is 1.4 V. The respective voltages for maximum current, Vm, are 1.9 V and 2.8 V. In Al2O3, the E.L. spectrum covers the visible range with peaks at 1.8 eV, 2.3 eV, and 4.0 eV. For Ta2O5, the E.L. intensity is constant over most of the visible, but has a maximum between 1.6 eV and 1.8 eV which is also the energy of E.L. of TaTa2O5Au diodes before the establishment of VCNR. For both Al2O3 and Ta2O5 diodes, electron emission into vacuum is anomalous since emission is detectable at diode voltages of ∼2 V. Electron emission in the two insulators, though qualitatively similar, shows differences that depend on differences in trapping levels and band gaps of the insulators. © 1972.