Many operating mechanisms of polymer light-emitting diodes (LEDs) are still not fully understood, among them the crucial step of recombination of oppositely charged carriers to form excitons. Questions related to this step are: What is the shape of the recombination zone and what is the quenching loss at the metal electrode layers? An investigation is reported in which the emission zone is directly determined by analyzing the polarization of the electroluminescence. It is shown that exciton quenching near the electrodes is a crucial parameter for the overall device efficiency.