The process of cyclisation via visible-light-mediated desulfurisation proceeds via formation of a cyclised peptide radical species which must undergo either H-atom abstraction or reduction and protonation to yield the desired product (Figure ). Since no excess thiol additive is present in the reaction mixture, remaining Cys (i.e. peptide) thiol would be an obvious H-atom source. However, this suggests that once initiated, the reaction should propagate without requiring irradiation. In reality, the reaction stalls in the dark or under ambient light. Moreover, no peptide degradation or epimerisation is observed over the course of the reaction. This indicates that the cyclised peptide radical is reduced by the Ir PC to the anion, then protonated to give the product and regenerate the oxidised catalyst.