Efficiency-limiting degradation of organic compounds is one of the main bottlenecks suppressing rapid commercialization of organic optoelectronic devices. Herein, degradation mechanisms of N,N'-di-1-naphthalenyl-N,N'-diphenyl [1,1':4',1'':4'',1‴-quaterphenyl]-4,4‴-diamine (4P-NPD) — a blue emitter, host for triplet harvesting and hole transport material commonly used in a variety of organic devices, are investigated. Controlled degradation tests reveal the material stability under individual influence of air or light exposure; however, rapid and irreversible degradation when simultaneously exposed to both factors. Degradation originating from photo-induced oxidation of 4P-NPD is characterized via fluorescence lifetime measurements. At a longer degradation time of ∼60 minutes, the fluorescence lifetime is found to stabilize. Exciton diffusion-based Monte Carlo simulations show that this fluorescence lifetime stabilization does not indicate the end of the degradation process. Defect concentration simulations, based on time-resolved measurements, show that the defect formation rate decreases as the degradation progresses. This self-inhibiting behavior indicates that the photo-induced oxidation proceeds through the excited state of 4P-NPD. The results provide insight into the degradation process of 4P-NPD, highly relevant for increasing the stability of 4P-NPD-based devices.