Based on literature precedent and focused experimental (Figure ) and computational data (Figure ), we propose the mechanism depicted in Figure . The [Ir III ] catalyst is excited by visible light to [Ir III* ]. Oxetane acid 1, which cannot be oxidized directly by [Ir III* ], is deprotonated by Cs2CO3 to 1 -, which is now oxidizable at +0.89 V vs SCE. 1 -transfers an electron to [Ir III* ] and is thereby oxidized to a carboxylate radical (not shown) that rapidly (k = 10 10 s -1 ) 48 decarboxylates to oxetane radical III. Oxetane radical III then undergoes an essentially irreversible (forward reaction ca. 500 times faster than reverse reaction) and slightly exergonic Giese addition into ethyl acrylate to form adduct 2a • , which is reduced by [Ir II ] to enolate 2a -, thereby regenerating the catalyst, [Ir III ]. In a final step, 2a -is protonated to form the product 2a. Deuteration studies (Figure ) support the protonation step, with high deuterium incorporation a to the ester with D2O as additive. The source of protons under the standard reaction conditions remains unclear, with no deuterium incorporated with either d-1 (COOD) or DMF-d7 and no significant effect of water content on the reaction outcome (Supporting Tables ). Di-addition product 2a' was shown to form predominantly via the polarity mis-matched radical conjugate addition of 2a • into a second molecule of ethyl acrylate (followed by reduction by [Ir II ] and protonation) and not by a more intuitive polar conjugate addition of 2a -into ethyl acrylate. 2a • can either directly add into ethyl acrylate after its formation, or it can be reformed at a later stage by oxidation of 2a -by the excited photocatalyst [Ir III* ], as suggested by the oxidation potentials (Figure ). Importantly, it was shown that the final product 2a is converted into 2a' by resubmission to the reaction conditions (18%), but not in the absence of [Ir], light; or base (<3%; Figure ). Slow depletion of 2a with increased amounts of 2a' were observed at long reaction times (Supporting Table and Figure ) . Oxidation potentials measured by cyclic voltammetry in a 0.1 M solution of NBu4ClO4 in MeCN at 25 °C with 100 mV s -1 scan rate and reported vs SCE. Anions generated in situ through the addition of 1 equiv NBu4OH (1 M in MeOH). Here, the anodic peak potentials (Epa) are reported. See the Supporting Information, page S35 for further details. Values calculated at the SMD 51 (DMF)-M06-2X 52 /ma-def2-TZVP//ωB97X-D3 46 /def2-SVP level (298.15 K, 1 M). Value for 2a • calculated using the methyl ester. d Values calculated using methyl acrylate at the CPCM 45 (DMF)-ωB97X-D3 46 /def2-TZVP//ωB97X-D3 46 /def2-SVP level (298.15 K, 1 M). e Yields and percentage of deuteration (D-%) calculated by analysis of the 1 H NMR spectrum of the crude mixture of the reaction using 1,3,5-trimethoxybenzene as internal standard and a 30 s relaxation delay (d1).