Abstract— Based on the observed changes in chemical compositions of fluorescent and phosphorescent carbazole-based OLEDs during operation, a free-radical mechanism of operational degradation is proposed. Chemical analysis and identification of low molecular weight and oligomeric products, device physics, photochemistry, and electron paramagnetic resonance (EPR) studies point to the excited-state homolytic-bond dissociation followed by radical additions as key mechanism steps. Comparable bond dissociation energy and singlet excited-state energy result in a relatively fast degradation process of carbazole-based OLEDs. OLED operation leads to the accumulation of solid-matrix-trapped long-lived π-radical species in their charged or neutral forms, acting as non-radiative recombination centers and luminescence quenchers. The proposed free-radicals-mediated degradation mechanism could be a common degradation mechanism affecting a wide range of OLED compositions and structures. In the framework of this mechanism, the relationship between the excited-state energy and the weakest bond dissociation energy of OLED materials is of the fundamental importance for the operational stability of OLED devices.