In conclusion, an assay has been employed to investigate the formation of aryl radicals using visible light-induced palladium catalysis for the coupling of aryl halides to arenes. Under photoexcitation, the assay confirms that various palladium sources and ligand combinations produce aryl radicals from aryl halides. The presence of radicals is confirmed both by diagnostic ratios of products 8:7 as well as by comparison of the outcomes of the reactions in deuterated versus undeuterated solvent. Additionally, LED-irradiation of the formal oxidative addition complexes, LnPd II ArI, is shown to produce aryl radicals. These findings highlight the ability of the assay to provide mechanistic information. The results of these photoactivated reactions contrast our previous study of ground-state palladium chemistry involving formation of biaryls from aryl halides and arenes under basic (BHAS) conditions. Under ground-state conditions, products are produced via a BHAS chain reaction requiring a strong base, while in photoactivated mode mild base instead promotes a Pd 0 /Pd I /Pd II catalytic cycle, supporting other reports on the reactivity of Pd-radical hybrid species.