Prebreakdown and breakdown electrical properties, and also the light emitted during destructive breakdown, have been studied in thin-film capacitors with the dielectrics SiO, MgF2, CaF2, CeF3, CeO2, and Teflon. Capacitors were formed on glass substrates and had dielectric thicknesses from 900 to 16000 Å. At temperatures between 80 and 380 K, breakdown characteristics are found to be independent of prebreakdown ac and dc conduction. All of the materials studied show a threshold field for the onset of breakdown (on the order of 106 V∕cm) that is almost temperature independent and varies with dielectric thickness w approximately as w−1∕2. All materials also have a threshold voltage for the cessation of breakdown which is typically between 10 and 20 V. The light emitted during breakdown contains the arc spectra of the dielectric and the two electrodes. Intensity of single wavelengths of light were recorded simultaneously with voltage waveforms and show that all lines are emitted from the very beginning of breakdown conduction. Hence, breakdown conduction is due to the formation of a gaseous arc. This arc, formed in less than 10−8 sec, carries the high current characteristic of breakdown and causes destruction of the dielectric.