The oxidation of calcium electrodes on polymer light emitting diodes (LEDs) can influence the lifetime of the LEDs by an order of magnitude. This finding illustrates how important the metal-polymer interface can be in LEDs. This aspect, which so far has usually been neglected, is investigated experimentally and interpreted in terms of quantum chemical modeling. It is shown that for all the metals considered (Al, K, Rb, Na, Ca) an interfacial region is formed between the metal and the polymer which behaves differently from the bulk.