The operational mechanism of polymer light-emitting electrochemical cells (LECs) in sandwich geometry is studied by admittance spectroscopy in combination with numerical modeling. At bias voltages below the bandgap of the semiconducting polymer, this allows the determination of the dielectric constant of the active layer, the conductivity of mobile ions, and the thickness of the electric double layers. At bias voltages above the bandgap, pn junction formation gives rise to an increase in capacitance at intermediate frequencies (approximate to 10 kHz). The time and voltage dependence of this junction are successfully studied and modeled. It is shown that impedance measurements cannot be used to determine the junction width. Instead, the capacitance at intermediate biases corresponds to a low-conductivity region that can be significantly wider than the recombination zone. Finally, the long settling time of sandwich polymer LECs is shown to be due to a slow process of dissociation of salt molecules that continues after the light-emitting pn junction has formed. This implies that in order to significantly decrease the response-time of LECs an electrolyte/salt combination with a minimal ion binding energy must be used.