Light‐emitting diodes made with poly(2‐methoxy‐5(2′‐ethyl)hexoxy‐phenylenevinylene) (MEH‐ PPV) using indium‐tin‐oxide (ITO) as anode and Ca as cathode have been examined as they age during operation in a dry inert atmosphere. Two primary modes of degradation are identified. First, oxidation of the polymer leads to the formation of aromatic aldehyde, i.e., carbonyl which quenches the fluorescence. The concomitant chain scission results in reduced carrier mobility. ITO is identified as a likely source of oxygen. The second process involves the formation of localized electrical shorts which do not necessarily cause immediate complete failure because they can be isolated by self‐induced melting of the surrounding cathode metal. We have not identified the origin of the shorts, but once they are initiated, thermal runaway appears to accelerate their development. The ultimate failure of many MEH‐PPV devices occurs when the regions of damaged cathode start to coalesce. ©1996 American Institute of Physics.