From all the processes of chemical self-purification of natural waters, radical processes have the greatest impact on the restoration of the initial natural chemical composition. The study of water self-purification processes is a complex task, which involves the study of both the chemical composition and the processes that determine this composition. Research in this field can allow the estimation of the maximum "load" with various pollutants of aquatic objects. The ecological state of waters can be evaluated with kinetic parameters: inhibition capacity, stationary concentration of hydrogen peroxide, redox state. However, these parameters do not suggest full information about the persistence of pollutants in the environment. The aim of the work is to estimate the impact on the processes of radical self-purification of waters by determining the antioxidant activity of some potential pollutants. Thus, two reducing compounds were selected, which are found in the water composition - cysteine (Cys) and ascorbic acid (AA). The antioxidant activity of the reducers was established by the DPPH method, calculating the EC100 parameter (the effective concentration of the substrate that consumes 100% oxidant (with reference to free radicals)), or the inverse of the antiradical power (ARP). EC100 values were obtained for ascorbic acid (0.74) and for cysteine (2.36), indicating a more pronounced antioxidant activity of AA (ARP=1.35) than Cys (ARP=0.42). In order to be able to interpret the obtained results, with reference to the impact of these reducers on the radical self-purification processes, they were analyzed in relation to the kinetic parameters of the induced photolysis of the studied compounds. According to them, ascorbic acid undergoes induced photolysis with a higher intensity (k=(2,45±0,09)‧10-4s-1) than cysteine, for which k=(1,77±0,10)‧10-4s-1. In other words, cysteine is more persistent in the environment and its oxidation requires a larger amount of free radicals. The EC100 values indicate a 3.2-fold lower antioxidant activity of cysteine compared to ascorbic acid, which can also be interpreted as cysteine being a reductant that requires higher amounts of oxidative equivalents to be oxidized.