Impedance spectroscopy (IS) of poly(9,9-dioctylfluorene) (F8) polymer light-emitting diodes (PLEDs) with hole-injection layers has been studied to determine their equivalent circuits. The equivalent circuits of the PLEDs below and above the threshold voltage for electroluminescence (EL) and their physical interpretation are shown. A significant difference in equivalent circuits is observed between below and above the threshold for EL. Above EL threshold, two R-L series branches, which are the origin of the negative capacitance effect, appear in the equivalent circuit. (C) 2008 Elsevier B.V. All rights reserved.