The valuable biological status of aquatic ecosystems is ensured by the dominance of oxidative equivalents, especially hydrogen peroxide and which is assessed by the parameter called water redox state. On the other hand, the group of reducing equivalents is represented by a wide range of compounds, including thiols. Due to the fact that the aquatic resources of the Republic of Moldova are limited and about 50% of the country‘s economy is provided with water needs from the cross-border river Dniester, there is a need to provide and protect it as a living environment for hydrobionts. Thus, the purpose of this research is to indirectly assess the redox state of the Dniester waters by determining the presence of peroxidase equivalents – thiol compounds. During 2021, the content of thiol compounds was monitored in the waters of the Dniester River, in the catchment area of Vadul lui Vodă. The detected concentrations of thiol compounds fall within the limits of the order of 10-6M, the annual average is 1,58·10-6M. To evaluate the provenance of thiol compounds in the Dniester waters, water samples were taken seasonally. Thus, the following concentrations were detected: in spring – 1,52·10-6M, in summer – 1,11·10-6M and in autumn – 2,12·10-6M. The detection of seasonal variations in thiols indicates their provenance. Thus, if they are products of the metabolic processes of hydrobionts, the concentration of thiols will be maximum during the summer, when the biological activity is high. Minimum concentrations will be in autumn and spring, periods in which biological activity decreases and chemical processes of self-purification of water bodies take place, including redox with the consumption of oxidative equivalents. Therefore, the seasonal variation was not detected, which indicates that the self-purification processes in the river waters during 2021 are slowed down and / or the source of thiol compounds is mixed, both natural and anthropogenic. Therefore, the redox state of the Dniester waters is unstable, characterized by the dominance of reducing compounds.