In polymer light emitting diodes (PLEDS) with an (ITO/PPV/Ca) structure we observed a significant reduction of both the current and the light output at constant voltage after heat treatment for only 30 min at 65 degreesC. Electroluminescence spectroscopy experiments showed that the shape as well as the amplitude of the spectra were changed. The reduction of current and light output was investigated by measuring I-V and E-V (current-voltage and brightness-voltage) characteristics of PLEDs, I-V characteristics of single carrier devices, and by performing low energy ion scattering and X-ray photoelectron spectroscopy experiments on the Ca/PPV interface. It was concluded that the current and light output reduction could be ascribed to the degradation of the Ca/PPV and the ITO/PPV interfaces. The degradation of the ITO/PPV interface resulted in a reduction of the zero field hole mobility and a small increase of the field dependence of the mobility. The degradation of the Ca/PPV interface, probably by diffusion of calcium into the PPV, resulted in carrier traps and quenching sites, which influenced the field dependent electron mobility. (C) 2003 Elsevier B.V. All rights reserved.