The principle of separating environmental factors was applied by using model sys-tems in laboratory conditions to determine the influence of humic substances (HSs) on the sensitized photolysis process of thiourea (TU). To achieve this goal, we modeled the following system: TU-HSs-hν. We subjected the model systems to successive irradiation using the DRT-400 polychromatic lamp and the Oriel Model 9119X Solar Simulator in the presence of the AM 1.5D filter. We carried out the calculation of the kinetic parameters for the sensitized photolysis of TU using the pseudo-first-order reaction, con-sidering the TU concentration. It was found that the degradation rates of TU depend on the irradiation source. The system irradiated with the DRT-400 polychromatic lamp shows sensitized photolysis rates of TU in the presence of HSs on the order of 10-7 M·s-1, while the system irradiated with SS in the presence of the AM 1.5D filter exhibits rates on the order of 10-8 M·s-1. This difference arises because HSs absorb radiation in the 220-365 nm range, with ab-sorption maxima at 254 nm and 365 nm. While the sensitized photolysis process with HSs under SS irradiation occurs at a slower pace, the emission spectra of this source nearly match those of sunshine. Therefore, the transformation patterns of TU under these conditions closely resemble those observed in natural aquatic systems. HSs, prevalent in natural aquatic systems, accelerate TU degradation when ex-posed to sunlight. These kinds of processes have a positive effect on the photochemical self-purification mechanisms of water, which helps the chemical properties in the aquatic environment get back to a state where living things can survive. This aligns with a fun-damental principle of aquatic ecological chemistry. In natural waters, TU undergoes transformations through intricate radical mechanisms, where photochemical reactions dominate primarily in the initial stages, followed by subsequent chemical reactions.