Triethanolamine is a widely used model electron donor that enables a fast screening of the photocatalyst parameters in both, homogeneous and heterogeneous scenarios. We report a new role of triethanolamine in heterogeneous photoregeneration of cofactor molecules - nicotinamide adenine dinucleotide (NADH) - using state-of-the-art heterogeneous photocatalysts. In contrast to the common model involving the light-induced electrons and holes generation to reduce the substrate and oxidize triethanolamine simultaneously, we identified glycolaldehyde as a stable product of triethanolamine degradation capable of reducing NAD(+). Triethanolamine, apart from playing a role of a precursor for reducing agent, maintains the alkalinity of the solution to drive the reduction. Our findings offer a fresh insight into the triethanolamine-assisted photocatalysis because glycolaldehyde as such have generally been neglected in mechanistic considerations. Moreover, a spatial and temporal decoupling of the photocatalyst from the substrate reduction reaction minimizes the product re-oxidation, thus implying a relevant feature for the real-world applications using a continuous flow setting.