AbstractCyanoarene-based photocatalysts (PCs) have attracted significant interest owing to their superior catalytic performance for radical anion mediated photoredox catalysis. However, the factors affecting the formation and degradation of cyanoarene-based PC radical anion (PC^•‒) are still insufficiently understood. Herein, we therefore investigate the formation and degradation of cyanoarene-based PC^•‒ under widely-used photoredox-mediated reaction conditions. By screening various cyanoarene-based PCs, we elucidate strategies to efficiently generate PC^•‒ with adequate excited-state reduction potentials (E_red^*) via supra-efficient generation of long-lived triplet excited states (T₁). To thoroughly investigate the behavior of PC^•‒ in actual photoredox-mediated reactions, a reductive dehalogenation is carried out as a model reaction and identified the dominant photodegradation pathways of the PC^•‒. Dehalogenation and photodegradation of PC^•‒ are coexistent depending on the rate of electron transfer (ET) to the substrate and the photodegradation strongly depends on the electronic and steric properties of the PCs. Based on the understanding of both the formation and photodegradation of PC^•‒, we demonstrate that the efficient generation of highly reducing PC^•‒ allows for the highly efficient photoredox catalyzed dehalogenation of aryl/alkyl halides at a PC loading as low as 0.001 mol% with a high oxygen tolerance. The present work provides new insights into the reactions of cyanoarene-based PC^•‒ in photoredox-mediated reactions.