Co I signature peaks disappear, and the original absorption profile of Co III complex is recovered (Figure ). Taken together, these results confirm the formation of a Co I intermediate that can react with a proton source to form a Co III -H or C2H2 directly to form a Co III -C2H2 adduct, suggesting that there are two possible mechanisms for acetylene reduction. We also observe a substantial decrease in evolved H2 from our system under 99.5% C2H2 atmosphere compared to Ar atmosphere (Figure ), suggesting again that H2 evolution and C2H2 reduction proceed via the same intermediate, and that C2H2 reduction outcompetes H2 evolution under our experimental conditions. It is well-established that photocatalytic H2 evolution with cobaloxime catalysts proceeds via a Co III -H intermediate, and consequently we hypothesized that C2H2 semihydrogenation also proceeds through a Co III -H intermediate. We observe no C2H2 reduction in the presence of the radical trap TEMPO (Figure ), which also implies the existence of a radical intermediate. The formation of the TEMPO-acetylene adduct was confirmed by LCMS analysis of the reaction mixture (Figure ). We conclude that C2H2 associates with the cobaloxime catalyst through hydrogen atom transfer (HAT) as opposed to migratory insertion, as HAT generates a radical intermediate and migratory insertion does not. While a Co III -C2H2 adduct may form, we believe that this is at most a minor pathway. Since no C2H4 production is observed in the absence of a proton source, we suggest that protonation of the Co I intermediate to generate a Co III -H is due to TFE as opposed to BIH in the full reaction mixture. While BIH can act as a proton donor once C2H2 is added, BIH is not a potent enough proton donor to react with Co I directly, which explains the persistence of Co I in Figure prior to the addition of C2H2. In saturated solution shows a (pseudo)reversible redox peak at -1.58 V (vs Fc/Fc + ) corresponding to the Co II/I couple (Figure ). The potential of the Co II/I reversible redox wave is shifted anodically by 45 mV with TFE added to the Ar-saturated solution of [Co(dmgH)2pyCl], which can be attributed to the equilibrium between Co I and Co III -H (Figure ). When the CV of [Co(dmgH)2pyCl] is recorded under C2H2, the addition of TFE to the electrolyte causes the rise in catalytic current coupled with the Co II /Co I reduction event (Figure ). Taken together, these results offer additional support to the formation of a Co III -H intermediate in the photocatalytic mechanism, and that TFE is the primary source of protons in the reaction.