Impurity sites in LEDs based on of Gai-xAlxAs for fiber-optic communication lines using thermal-stimulated current method were researched. The causes of the degradation of such diodes are clarified. The installation for research using the method of thermal-stimulated currents is described. Given in p-n junctions based on GaAlAs. The dependence curves of thermal-stimulated currents and temperature are shown at various heating rates. Was made a research of light-emitting diodes degradation by their power supply of current pulses up to 10 A, with a duration of 100 ns and a frequency of 300 Hz, as well as at 50 mA, 20 mA and a temperature of 800C. A connection was found in the process of degradation of LEDs with an increasing of the concentration of impurity sites. The curves of thermal-stimulated currents determined the concentration of impurity sites before and after the degradation of LEDs. It is shown that the main reason for the change in the electrical characteristics of the p-n junctions of the studied samples upon passing a direct current is the accumulation of impurity sites.