Iron and cobalt porphyrins (FeP and CoP) are utilized as electron-transfer mediators to effect photochemical reduction of CO2 in homogeneous solutions. The species that activate and reduce CO2 are the (FeP)-P-0 and (CoP)-P-0 formed by reduction of the starting materials. Reduction of the metalloporphyrins (MP) is achieved by photolysis in dimethylformamide or acetonitrile solutions containing triethylamine (TEA) as a reductive quencher. The photoreduction is efficient for the (MP)-P-III --> (MP)-P-II stage and probably occurs by an intramolecular electron transfer from an axially bound TEA. However, TEA does not bind to the reduced metal complexes, and the quantum efficiency is much lower for the subsequent reduction steps. Considerably higher quantum yields are obtained by adding p-terphenyl (TP) as a sensitizer. TP is very effectively photoreduced by TEA to form the radical anion, TP.-, which has a sufficiently negative reduction potential to reduce (CoP)-P-I and (FeP)-P-I rapidly to their MOP state. The rate constants for these reactions, determined by pulse radiolysis, are found to be nearly diffusion-controlled. The quantum yield for the reduction of (MP)-P-II to (MP)-P-I and for reduction of CO2 to CO are increased by more than an order of magnitude in the presence of TP. Side reactions involve hydrogenation of the porphyrin ring and production of Hz. The hydrogenated porphyrins also catalyze reduction of CO2, but the photochemical production of CO eventually stops. This limit on catalytic activity is due to destruction of the porphyrin macrocycle and accumulation of CO. CO can bind strongly to (FeP)-P-II and to (FeP)-P-I but not to (FeP)-P-0, as demonstrated by electrochemical measurements and by optical spectra of the species produced by sodium reduction in tetrahydrofuran in the presence and absence of CO. Although binding of CO to (FeP)-P-II and FeIP should not interfere with the formation of (FeP)-P-0, the active catalyst, the potential for reduction of (FeP)-P-I to (FeP)-P-0 becomes more negative. However, CO probably binds to the hydrogenated products thereby inhibiting the catalytic process.