Cyclic voltammetry (CV) studies have been carried out on polymer light-emitting electrochemical cells (LECs) using poly(2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, as the active luminescent polymer. As a result of the slow ionic motion in response to changes in the applied voltage, the CV scans show hysteresis. The onset oi electronic current in an LEC is coincident with the onset of electrochemical doping and the formation of a p-n junction. From analysis of the measured current and light emission during cyclic voltammetry, we conclude that the voltage threshold for the onset of electrochemical doping in MEH-PPV is approximately 2.2 V.