On the example of vitamins of the B complex (B6 and B9), in concentrations that model the pollution situation in natural waters, research was carried out to establish the load of pollutant (vitamin) that can be accepted by natural waters, so that they maintain the self-purification capacity. The researches were carried out on microcosm type systems that contained waters of Lake Dănceni. The self-purification processes were monitored according to the following parameters: pH, degree of oxygen saturation, permanganateometric oxidizability, inhibition capacity. The results of the modeling of the self-purification processes of the natural aquatic systems demonstrated that when the waters of the Dănceni lake are polluted with vitamins of the B complex, in concentrations above 0.8·10-4 M, the self-purification capacity of the natural aquatic system, through chemical oxidation processes, is exceeded.