Kinetic studies were conducted to gain more insight into the mechanism of oxidative addition of Bi(I) into the C-I bond of aryl iodides. Reactions were carried out in an NMR spectrometer for in situ monitoring of the reaction progress, using the fiber optic-coupled LED setup as previously described by Gschwind and coworkers (see Section 5.1 in the Supporting Information for more experimental details). Initial rates were obtained from the plotted concentration profiles at ≤ 10% conversion. To accurately mimic the experimental conditions during Kessil light irradiation, a constant temperature of 35 °C was maintained during the in situ LED-NMR time courses using the active temperature control capabilities of the NMR instrument (see Section 5.1 in the Supporting Information). The rate of oxidative addition of 1a into 4-iodobenzonitrile exhibits a first-order dependence on the light intensity and the aryl iodide concentration (Figures and, and Sections 5.3 and 5.4 in the Supporting Information). Somewhat surprisingly, the initial rates were independent of the Bi(I) concentration within the range of 2-33 mM (Figure and Section 5.5 in the Supporting Information). The latter can be rationalized by invoking a light-limited formation of the excited-state bismuthinidene, likely due to the high molar absorptivity for the wavelength of light used (ε = 3620 M -1 cm -1 @ 630 nm). Overall, the kinetic dependence on the various reaction components suggests a rate-limiting interaction of the excited-state bismuthinidene and the aryl iodide. A Hammett plot was constructed from initial rates of oxidative addition with a series of para-substituted aryl iodides (Figure ). Plotting the initial-rate data vs standard Hammett σp values gave the best fit of the data, revealing a positive correlation ρ = 1.5. This is consistent with a net buildup of negative charge on the aryl iodide in the rate-limiting step. Furthermore, the linearity of the plot also suggests no change in mechanism across the series of aryl iodides tested. Although smaller ρ values have typically been used to support the existence of XAT over SET-based mechanisms for oxidative additions at transition-metal centers, we hypothesized that an attenuated ρ value could also arise from a concerted dissociative electron transfer mechanism or a stepwise mechanism featuring rate-limiting contributions from both electron transfer and mesolytic cleavage steps (whose rates will increase and decrease, respectively, for more electron-deficient arenes). With this in mind, we sought to probe this mechanistic dichotomy further by conducting a series of intra-and intermolecular competition studies.