The lifetime of a phosphorescent organic light-emitting device (PhOLED) may range over several orders of magnitude depending on the choice of the phosphorescent emitter guest material and its concentration. This behavior, we find, results primarily from different rates of exicton–polaron-induced aggregation (EPIA) of the host material due to exciton–polaron interactions. We investigate the influence of singlet versus triplet excitons in the EPIA of host materials and the degradation of PhOLEDs. The results show that there is a correlation between device electroluminescence degradation and the concentration of singlet excitons on the host during electrical driving. In contrast, we find no particular correlation between device degradation rate and the concentration of triplet excitons. The results therefore reveal that singlet excitons play the leading role causing the EPIA of host materials and device degradation by this mechanism.