Following the discovery that conjugated polymers can be used as the emitting layer in light­ emitting devices, extensive studies on these materials have been carried out. In a few years, due to the development of new materials and an increasing knowledge of the underlying chemistry and physics, the performance has improved a lot. One of the important factors that determine the performance of the devices is the chemistry upon interface formation. In the simplest single-layer device there are two interfaces, the interface between the conjugated polymer and the anode and cathode respectively. This thesis deals to a large extent with investigations of the chemical and electronic structure at these interfaces by means of X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The first chapters give a short introduction to conjugated polymers, polymer LEDs and photoelectron spectroscopy on conjugated material. The following three chapters provide some background information in the subject areas of the papers with some references to closely related studies. The first paper compares the device performance characteristics upon using two different anodes, indium tin oxide (ITO) and fluorine tin oxide (FTO). The second and third papers report on chemical interactions between the ITO-anode and the precursor polymer of PPV, one of  the most frequently used conjugated polymers. The fourth and final paper reports on the early stage of interface formation between two different metal atoms (lithium and sodium), and a conjugated molecule.