Cobalt corrins (B-12) are found to act as homogeneous catalysts for photoreduction of CO2 to CO and formic acid Photoreduction is carried out in acetonitrile/methanol solutions containing p-terphenyl as a photosensitizer and triethylamine as a reductive quencher. Photolysis (lambda greater than or equal to 300 nm) leads to production of CO and formic acid as well as H-2. The rate of production of all three products is considerably higher with the corrins (hydroxocobalamin, cyanocobalamin, and cobinamide) than with cobalt tetra-m-tolylporphyrin. The mechanism of CO2 reduction in all cases is via a species formed by one-electron reduction of the Co(I) complex. Radiolytic studies of the Co(I) complex formed from hydroxocobalamin in aqueous solutions show that this compound reacts very rapidly with solvated electrons and more slowly [k = (1.2 +/- 0.3) x 10(8) L mol(-1) s(-1)] with CO2.- radicals to produce different products. The initial reduction product is suggested to be mainly the hydride formed by protonation of a Co(0) corrin, (HCoB12)(-). The product of reaction with CO2.- is suggested to be the adduct (CO2CoB12)(2-) or its protonated form (HO2CCoB12)(-), identical to the adduct formed by reaction of CO2 with the photochemically reduced Co(I) corrin, which proceeds to produce CO. H-2 is formed by reaction of the hydride with a proton. Side reactions leading to hydrogenation of the macrocycle also take place and limit the catalytic activity.