The mechanism of CO 2 reduction and hydrogen production by a homogeneous photocatalytic system consisting of a copper (Cu) complex as a photosensitizer and an iron (Fe) spincrossover complex as a catalyst was investigated using density functional theory. The photocatalytic reaction is initiated by electronic excitation upon light irradiation, characterized by a charge transfer from the metal d-orbital to the ligand orbital of the Cu photosensitizer (Cu PS). The long-lived lowest energy triplet state exhibits a larger oxidation power than the ground state, and electron transfer from the sacrificial electron donors results in the formation of a reduced species of Cu PS. The electron transfer from the one-electron reduced species of Cu PS to the Fe complex leads to CO 2 reduction. The calculations suggest that the two-electron reduction of Fe complexes is necessary for CO 2 reduction and that the complexes with CO 2 bound to the Fe center are the target species for the second electron reduction, but the oneelectron reducing species of Cu complexes has a weak reducing power and the Fe complexes are not reduced efficiently. For the subsequent catalytic reaction, the binding of CO 2 to the Fe center was substantially endergonic, and the binding stability was low. Therefore, in order to increase the catalytic efficiency, a ligand design that improves the reducing power for the Cu complexes and the CO 2 binding stability to the Fe center for the Fe complexes is essential.