In summary, we have mapped out the excited state nature and dynamics of red light-active bismuthinidene 1a and elucidated the mechanism by which the key MLCT excited state can effect net oxidative addition into aryl iodides. The photophysics were mapped out using fsTAS, revealing a red-light absorption attributed to a S0 → S1 transition, which is promptly followed by an ISC and vibrational cooling to populate an emissive triplet excited state. The ability of the Bi complex to undergo ISC with dissipative losses as low as 6% of the energy of the incoming light is noteworthy in the context of light harvesting strategies. The nature of the excited state was determined using CASSCF/NEVPT2, which calculated a pz-MLCT state with radical anion character on the ligand and radical cation character on bismuth. Spectroelectrochemical measurements support Bi(II) •+ character in the excited state. For the investigation of the mechanism of oxidative addition into aryl iodides, kinetic studies revealed a first-order dependence on light intensity and aryl iodide concentration, and a zeroth order dependence on the concentration of 1a, indicating that the formation of the excited state under the reaction conditions is in a light-limited regime. Intra-and intermolecular competition studies allowed us to dismiss inner-sphere pathways for the activation of the aryl iodide (either via XAT or inner-sphere electron transfer) and suggest an outer-sphere SETbased activation of the aryl iodide as the operant mechanism. The combination of the kinetic and competition studies suggests a complex rate law, where the rate of the reaction is influenced by both the reduction potential of the aryl iodide, as well as the rate of mesolytic cleavage of the C-I bond from the corresponding radical anion. We anticipate that this mechanistic understanding of excited-state reactivity will act as a foundation for the development of next-generation red light-active bismuth photocatalysts and aid in expanding the synthetic repertoire of bismuth redox catalysis. Furthermore, the recognition of the low dissipative losses incurred during ISC will spur efforts to explore the application of related bismuth complexes for light-harvesting energy conversion and photon up-conversion processes. Such efforts are currently underway in our laboratories and will be reported in due course.