The effect of controlled exposure to O on the stability of Ca cathode of polymeric light emitting diodes (pLEDs) was studied. The LEDs were fabricated with ITO as anode, poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] [OC1C10-PPV] as electroluminescent polymer, Ca as cathode and Al as protecting layer. The polymer layers of the LEDs were spin coated under dry N atm. and transported directly into an UHV chamber where the metal electrodes were deposited by evapn. To study the effects of exposure to O of the Ca cathode, deposition of the Ca layer was interrupted in some cases and the samples were exposed to 30-1000 mbar of O. The amt. of O in the layers of the pLEDs was detd. from current-voltage data and elastic recoil detection and was correlated with device performance parameters. Exposing a part of the Ca layer to O at layer thicknesses equal to or less than 10 nm led to a total loss of brightness. The failure of pLEDs is attributed to oxidative degrdn. of OC1C10-PPV in contact with oxidized Ca and O at the interface. The damage and failure can be prevented if fabrication of the devices is carried out in a totally inert atm