We have studied the electrical fields in single-layer polymer light-emitting diodes (LEDs) using electroabsorption (EA) spectroscopy. The materials under investigation were PPV derivatives, namely poly(2,5-dialkoxy-para-phenylene vinylene) and a precursor route poly(2-methoxy-5-ethyl(2′-hexyloxy)-para-phenylene vinylene). The applied bias generates an opposing electric field that decays over time. The magnitude of the reversed field increases as the applied DC bias is increased and can be as high as one-third of the applied external field. The development of the internal field is less pronounced in vacuum than in air and it vanishes for devices prepared and tested under inert conditions.