Triethanolamine (TEOA) has been used for the photocatalytic reduction of CO, and the experimental studies have demonstrated that the TEOA increases the catalytic efficiency. In addition, the formation of a carbonate complex has been confirmed in the Re photocatalytic system where DMF and TEOA are used as solvents. In this study, we survey the reaction pathways of the photocatalytic conversions of CO to CO + HO and CO to CO + HCO by -Re(bpy)(CO)Br in the presence of TEOA using density functional theory (DFT) and domain-based local pair natural orbital coupled cluster approach, DLPNO-CCSD(T). Under light irradiation, the solvent-coordinated Re complex is first reduced to form a monoalkyl carbonate complex in the doublet pathway. This doublet pathway is kinetically advantageous over the singlet pathway. To reduce carbon dioxide, the Re complex needs to be reduced by two electrons. The second electron reduction occurs after the monoalkyl carbonate complex is protonated. The second reduction involves the dissociation of the monoalkyl carbonate ligand, and the dissociated ligand recombines the Re center carbon to generate Re-COOH species, which further reacts with CO to generate tetracarbonyl complex and HCO . The two-electron reduced ligand-free Re complex converts CO to CO and HO. The pathways leading to HO formation have lower barriers than the pathways leading to HCO formation, but their portion of formation must depend on proton concentration.