Both intrinsic and extrinsic degradation mechanisms limit the long-term stability of lightemitting diodes (LEDs). Here the nature of black spots formed on such devices is examined in detail. It is concluded that black spots, which form around pinhole defects (see Figure for mechanism of pinhole defect formation at grain boundaries), cause a reduction of total luminance output but not of device efficiency. [GRAPHICS] Nature of Non-emissive Black Spots in Polymer Light-Emitting Diodes by In-Situ Micro-Raman Spectroscopy.