Efforts towards probing the mechanism of oxidative addition started by differentiating between concerted (SNAr-type) and stepwise (SET, XAT) pathways. Since the latter pathways feature aryl radicals as obligate intermediates, we conducted the oxidative addition of 1a into 1-(allyloxy)-2-iodobenzene 4 as an aryl radical trap (Figure ). Under red light irradiation, the reaction afforded a ca. 1 : 8 mixture of the non-cyclized (5a) to cyclized (5b) oxidative addition products. It is worth noting that the known stability of N,C,N-Bi-aryl species under red light irradiation precludes conversion of 5a to 5b. This result strongly supports the existence of an aryl radical intermediate during the mechanism of formal oxidative addition. Since the nascent aryl radical derived from this substrate is known to undergo a 5-exo-trig cyclization with a rate constant of 9.6 × 10 9 s -1 , the ratio of products allowed us to clock the effective first-order rate constant of radical recombination between the solvent-caged aryl radical and the putative Bi(II) radical at ca. 1.7 × 10 9 s -1 , which is likely approaching the diffusion limit. Additionally, UV-Vis and 1 H NMR experiments do not suggest the formation of a photoactive EDA complex between 1a and 2 (see Sections 8.1 and 10.3 in the Supporting Information), thereby strongly suggesting that the observed reactivity is arising from an interaction of the excited MLCT state of 1a with the aryl iodide. Given the above data, two main stepwise mechanistic scenarios were considered. The first scenario invokes single-electron transfer from the bismuthinidene excited state to the aryl iodide (Figure , path a). Given the redox potentials of the excited state (E°(Bi II/* ) = -2.62 V vs Fc +/0 ) and the aryl iodide (Ep/2 ≈ -2.10 V vs Fc +/0 ), this amounts to an exergonic electron-transfer event (12 kcal mol -1 ), which most likely falls in the Marcus normal region. The downstream productive mesolytic cleavage competes with back-electron transfer (BET), which has a driving force of ca. 29 kcal/mol (E1/2(Bi II/I ) = -0.85 V vs Fc +/0 ) and returns the ground state bismuthinidene and neutral aryl iodide. It is worth noting that the dissociative electron transfer can also occur in a concerted fashion. The second main mechanistic pathway considered involves a concerted, irreversible halogen atom transfer (XAT) event between the excited-state bismuthinidene and the aryl iodide (Figure , path b). Unlike the first mechanistic scenario, the XAT pathway would not be subject to competing back-electron transfer events.