Charge transport in light-emitting diodes (LEDs) based on a poly (p -phenylene vinylene) (PPV) derivative is investigated as a function of sample thickness. Via the thickness dependence, the contributions from the electric field and charge carrier density to the mobility in space-charge-limited (SCL) diodes can be disentangled. It is demonstrated that a field-dependent mobility weakens the thickness dependence of the SCL current, whereas a carrier-density-dependent mobility gives rise to an enhanced thickness dependence. The enhanced thickness dependence of the experimental SCL current in PPV is in agreement with the predictions using a density-dependent mobility only. This observation confirms that in PPV-based LEDs, the hole transport is dominated by filling of the localized states.