We present a study of reliability of AlGaN-based 280 nm deep ultraviolet light-emitting diodes on sapphire substrate grown by migration-enhanced metal-organic chemical vapor deposition. Two modes of optical power degradation were observed: catastrophic and gradual. The catastrophic degradation is believed to be due to metal alloying at macroscopic defects in the top p layers of the light-emitting diode structure. For the gradual power degradation, two time constants were determined, which were temperature and bias dependent. For the temperature-dependent part, the values of the activation energies and room-temperature degradation rates at dc currents of 10 and 20 mA were determined to be 0.23 and 0.27 eV and 1.31×10-3 and 5.93×10-3 h-1, respectively.