The performance of light-emitting electrochemical cells is studied under steady-state conditions. The theoretical model presented is based on the approach to electrodiffusion-reaction problems familiar with electrochemical cells. However, some links with the traditional semicondutor physics approach are also presented. The reversible redox electrode reactions are considered to always be in thermodynamic equilibrium, and the distribution of the applied potential between the two electrode/polymer interfaces and the active layer is carefully analyzed by the numerical solution of the transport equations. The theory is able to explain most of the experimental observations. Ln particular, the junction formation is proved to be a bulk process related to the electrodiffusion and recombination reaction of polarons, thus explaining its macroscopic nature. Current-voltage and light-voltage characteristics in qualitative agreement with experiment are found. The observed changes in the junction width and the electric field profile with applied voltage are also accounted for. Some ideas for future work are also discussed.