Organic light-emitting diodes show increasing promise for technological application. Polymer based systems, although offering some processing advantages over LEDs based on organic small molecules, have not yet demonstrated sufficient stability for commercial devices. We have therefore undertaken a study aimed at identifying the degradation mechanisms in LEDs fabricated from a single layer of MEH-PPV between ITO as transparent anode and calcium as cathode. Current-voltage, luminance, spectroscopy and in-situ IR measurements were made. The results reveal several processes leading to reduced power conversion efficiency and eventual failure. Chemical degradation of the polymer, even in devices protected from ambient oxygen, both reduces its ability to transport charge and introduces carbonyl moities which quench the luminescence. FTIR spectroscopy provides details of the chemical processes involved and indicates that ITO can act as an oxygen reservoir in the formation of aromatic aldehydes. Additionally, inhomogeneous degradation occurs by the formation of "hot-spots" which shunt current from the remainder of the device area. Such areas may eventually become electrically open, perhaps due to thermal runaway, thereby reducing the active area of the diode.