The short operating lifetime of polymer light-emitting devices (PLEDs) is currently the main obstacle preventing their use on a commercial scale. In this study, the effect of the operating electric field on the degradation of PLEDs, made of poly(p-phenylenevinylene) as the emitting layer, is investigated using ac impedance spectroscopy and photoluminescence spectroscopy. It is found that, under the effect of the field, the interfaces between the polymer and the electrodes are greatly changed, mainly due to the electrochemical dissolution of the metal electrodes. This leads to the failure of the PLED by changing its conduction mechanism from the originally electronic one, which is necessary to produce the radiative relaxation of electron-hole pairs, to one that is essentially of ionic nature and eventually leads to electrode failure due to corrosion.