Monitoring the catalytic reaction by in situ NMR spectroscopy was carried out to gain insight into the cobalt-catalyzed reductive hydroformylation. Notably, all spectroscopic measurements were performed in the absence of blue light irradiation. At 30-50% con version of 1-octene, the benzene-d6 31 P NMR spectrum exhibited the resonance for Co-2 (δ 102.54) along with two new peaks centered at 88.7 and 83.1 ppm, assigned as the formate complex (dcype)CoC(O)OC9H19, arising from CO migratory insertion into the putative cobalt alkoxide. When the catalytic reaction reached complete (>99%) conversion, only Co-2 was observed by 31 P NMR spectroscopy. To determine if both cobaltcatalyzed hydroformylation and aldehyde hydrogenation are photodriven, 3-phenylbutyraldehyde was cleanly converted to 3-phenylbutanol using 5.0 mol% Co-2 under all mixtures of CO/H2 used in this study with blue light irradiation. Notably, aldehyde reduction was not observed under thermal conditions, illustrating that visible light is required for both steps of the tandem catalytic process. Under varied H2 pressures (1-4 atm), visible-light-mediated aldehyde reduction did not occur. The origins of the high regioselectivity in the photodriven cobalt-catalyzed reaction were investigated by subjecting vinyl epoxide (butadiene monoxide) to the optimized catalytic conditions for terminal alkenes (Scheme 2A). Monitoring the progress of the reaction by uct of 2,1 olefin insertion (Scheme 2B). The observation of the 4-carbon allylic alcohol supports a pathway where for alkenes lacking vinyl epoxides, 2,1-insertion is reversible and does not lead to aldehyde formation. The isolation of a bis(phosphine)cobalt acyl complex that underwent CO deinsertion under visible-light irradiation was recently reported that occurred even in the presence of excess syngas (vide supra). Taken together, these observations support that hydroformylation under visible light generates linear aldehydes exclusively -a consequence of rapid interconversion of linear and branched alkyls under visible light with acyl hydrogenolysis occurring exclusively from the linear isomer. In summary, a cobalt-catalyzed tandem hydroformylation/hydrogenation mediated by visiblelight has been realized. The method has been applied to the synthesis of C1-homologated alcohols from a variety of terminal and 1,1-disubstituted alkenes with exclusive linear selectivity. Mechanistic studies support reversible olefin insertion under visible light, where only linear aldehydes are released in the first of two concurrent transformations in the catalytic reaction.