Organic light emitting diodes (OLED) are efficient light sources based onorganic semiconductors. Unlike inorganic LEDs which are more or less pointsources, OLED are planar light sources with up to 1 m2 in area.By using organic materials, they are cheap to produce and economical to use.The determination of triplet exciton energy levels is of interest for thedevelopment of efficient OLED, based on the fact that electrical excitationusually creates three times as many triplets as singlets. Additionally, theknowledge of these energy levels is crucial for the design and choice ofemitter matrix materials and exciton blocking layers. These values arenormally determined by photoluminescence (PL) measurements in solution formaterials which show intersystem crossing (ISC) between singlet and tripletstates. For some materials, the triplet levels cannot be measured this waybecause some materials prohibit ISC. In this work, a method is presentedwhich allows the determination of the energy levels using low-temperatureelectroluminescence (EL) spectroscopy. The dependence on ISC is avoided bycreating triplets directly with electrical excitation and this allows tomeasure a large class of organic materials. A low-temperature EL spectrum ispresented forN,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (TPD) ina 3-phenyl-4-(1‘-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) matrix (TPD/TAZ1:3) at 77 K. Triplet emission is only observed at very low charge carrierdensity (0.5 μA/mm2). Quenching processes are analyzed usingcombined EL and PL measurements and unipolar devices. Two factors can be thecause of the quenching: A strong quenching based on a low concentration ofelectrically activated impurities could explain the dependency. The otherexplanation points to a quenching based on electrons in the emitting layer.This might be explained with triplet-polaron quenching (TPQ). TPQ isproportional to the charge carrier density and contributes the dominant partto the quenching at low current densities.