Sulfur derivatives of biogenic origin, such as some of the thiols, have strong reducing properties and actively participate in redox processes in the natural aquatic objects. On the example of cysteine and glutathione, thiol derivatives, which are formed as a result of biochemical processes inside of the water bodies and waters of Danceni Lake, from the Dniester river basin, has been conducted the researches to determine the load of substance that will not hinder the development of self-purification processes that form self-purification capacity of the natural aquatic system. The research was conducted on microcosm systems, using lake water from the summer season. Thiol concentrations were selected in such a way as to model the pollution situation in natural waters and were of the 10-4 M order. The monitoring of self-purification processes was performed according to the parameters: degree of oxygen saturation (GS), permanganatometric oxidability (CODMn) and inhibitory capacity (Σki∙[Si]). Research has shown that both thiols, in concentrations of the order (0.8-2.0)·10-4 M, affect the self-purification capacity of the natural aquatic system, by reducing it, because it consumes in its own transformation processes oxygen and its active forms from the water, us hydrogen peroxide and OH radicals. At the same time, it was found that the chemical-biological self-purification system of Lake Danceni neutralizes the negative effect of „redox-toxicity‖, caused by the presence of thiols in concentrations of the 10-4 M order, for a maximum 48 hours.