Electrical treeing is often responsible for the breakdown of insulating materials used in power apparatus such as high‐voltage transformers, cables, and capacitors. Insulation, such as crosslinked polyethylene (XLPE), used in underground high‐voltage cables usually operates at temperatures above ambient. This paper describes the characteristics of luminescence, emitted prior to electrical tree inception, at a crosslinked polyethylene‐semiconducting material (XLPE‐semicon) interface held above room temperature. Use of a sensitive light detection system showed that XLPE subjected to elevated temperatures emits luminescence even without voltage application. This light was attributed to thermoluminescence which decreased with the decrease in the concentration of the crosslinking by‐products present in the polymer. The spectra of thermoluminescence were only in the visible range. On the other hand, electroluminescence occurred when the XLPE‐semicon interface was held above room temperature and subjected to high electric stress. This light did not depend on the concentration of the crosslinking by‐products and the spectra of electroluminescence were in the visible and the ultraviolet ranges. It is proposed that XLPE‐semicon interface held at elevated temperature and subjected to long‐term voltage application initially emits both thermoluminescence and electroluminescence. As the crosslinking by‐products exude out of the polymer, thermoluminescence decreases with time and ultimately ceases, but electroluminescence occurs as long as the voltage is applied to the polymer. Although the intensity of electroluminescence emitted at high temperature was lower than that emitted at ambient, sufficient ultraviolet radiation was emitted. The ultraviolet radiation could photodegrade the polymer and lead to electrical tree inception.