A number of factors in this study contrast with our earlier assay results under ground-state conditions. The most obvious of these is the versatility of base that can be employed, whereby under photoactivation, radical reactivity is not limited to reactions using potassium alkoxide bases such as KO t Bu. Additionally, base is not required for high yields in stoichiometric reactions with oxidative addition complexes. These findings indicate that products are not generated by a BHAS chain reaction and instead base is involved in regenerating Pd 0 from HPd II X intermediates as part of a catalytic cycle. This proposal aligns more closely with literature precedent. Potassium alkoxide bases are thought to be essential in BHAS chemistry due to (i) high basicity to deprotonate the cyclohexadienyl radical intermediate and (ii) favourable ion- interactions between the potassium cation and haloarenes to lower reduction potentials. We did consider whether photoexcitation might increase acidity of intermediates to aid deprotonation and/or increase the reducing power of electron donors, allowing alternative weaker bases, like Cs2CO3, to facilitate coupling under a BHAS chain mechanism. To investigate this, a biphenyl radical anion (the type of electron donor involved in propagation of a BHAS cycle) was generated by reaction of 4,4′-di-tert-butylbiphenyl (DDTB) 15 with 1 equiv. of potassium metal in THF, generating a dark blue/green solution indicative of formation of the radical anion 16. 45 The 16/THF solution was then added as an initiator (ca. 17 mol%) to the assay reaction under different conditions (Table ). When added to a reaction using KO t Bu as base at 130 °C, i.e. our normal thermal conditions under which BHAS is expected to cycle, consumption of 6 was ca. 45%, indicating cycling of the chain reaction (entry 1). Although the major product was 9, as expected from the introduction of THF (bearing weak OC-H bonds) to the reaction mixture, yields of 7 and 8 were higher than those in the absence of initiator. When added to a reaction with Cs2CO3 as base either under thermal conditions (entry 2) or photoexcited conditions (entry 3), consumption of 6 was ca. 23% which, within error, does not indicate propagation of a BHAS cycle. Accordingly, any role for Cs2CO3 under photoexcitation as a base in a BHAS chain mechanism can be ruled out. Based on these experiments it is likely that the reaction of aryl halides with arenes under visible lightinduced palladium catalysis is facilitated through a Pd 0 /Pd I /Pd II cycle, a plausible mechanism for which is outlined in Scheme 2. Initially, a Pd 0 species generated in situ is photoexcited to Pd 0 *. This then undergoes SET with iodoxylene 6 yielding Pd-radical hybrid 17, which then has two paths of reactivity. In Path A, the radical attacks the * system of benzene, forming cyclohexadienyl radical hybrid 18, which reversibly recombines with Pd I to form 19, and after -hydride elimination yields dimethylbiphenyl 7 and Pd II HX, from which base regenerates Pd 0 . Alternatively, through Path B, 17