The dielectric breakdown of polyethylene subjected to a highly divergent electric field has been studied. The samples were held at room temperature and an alternating field of frequency 50 Hz applied. When the material contained only residual gas it was possible to detect light emanating from the bulk of the polymer. It is proposed that this was an electroluminescence of the polyethylene resulting from the recombination of charge carriers injected at the electrode–polymer interface with the deep trapping centers in the polyethylene. The detection of this light required the use of a very sensitive photomultiplier coupled to an optical microscope. When the material contained a dissolved gas in equilibrium, the behavior of the solid was found to depend on the chemical reactivity of the gas and on its electron affinity. In particular, the influence of oxygen has been studied. A model is proposed which accounts for all the experimental results and which underlines the fundamental role played in the prebreakdown process by the gas dissolved in the polymer.