Measurements of the frequency dependence of the complex admittance are used to separate the ionic and electronic contributions in polymer light-emitting electrochemical cells (LECs). At zero bias, a large polarizability, attributed to salt molecules in the electrolyte, is observed at low frequencies. Ions are generated when the LEC is biased at voltages above the threshold for electrochemical redox doping. Because of the slow ionic response, a novel pulsed drive scheme is proposed: the mean value stabilizes the inducedp-i-njunction, while the peak value controls the carrier injection and the brightness of the electroluminescent emission. LECs operated in this way exhibit fast emissive response and improved operating life. ©1998 American Institute of Physics.