Oxidation-reduction processes in the natural waters involving hydrogen peroxide, as carrier of reaction-capable oxidizing equivalents, and compounds of the reduced sulfur as carrier of reducing equivalents are considered. It is shown that a dominance of inflow of reductive substances that stable to O2, but efficiently interact with H2O2 leads to formation of a toxic quasi-reductive state of the natural water environment. Concentration of oxygen in water at the quasi-reductive state can remain normal. Change of a dynamic redox-state of a natural aqueous medium is pernicious for aquatic organisms with intensive water-exchange with the external environment, in particular for larvae of fishes. Favorable conditions for pathogenic microorganisms, "flowering" of blue-green algas, emergence in water of alga-toxins are formed. When quasi-reductive state is formed in natural water ions of copper become biologically inaccessible. Toxic properties of an aqueous medium are shown at quasi-equilibrium concentration of the reducing equivalents >10-7 g-eqv./l. Definition method of these reductive substances is based on titration of water sample by small additives of hydrogen peroxide in the presence of peroxidase and a leucio-base of trimetylphenolic dye. Sewage after their biological cleaning are the main anthropogenic source of the reductive substances in the form of derivates of hydrosulphide. As natural sources of the reductive substances in a natural aqueous medium serve blue-green algas and the bottom sediments saturated with organic matter. It is shown that ions of copper (I) and iron (II) form high-strength complexes with reduced sulfur compounds of structure 1:1 that are stable to O2, but efficiently interact with H2O2. Oxidation-reduction processes with participation copper ions and hydrogen peroxide in a neutral aqueous medium are followed by formation ∙O2- and OH free radicals. The free radicals can make an essential contribution to a self-cleaning of a natural aqueous medium from organic pollutants. However, at an excessive intensification of free-radical processes, at quasi-steady state concentration of OH radicals more than 10-15 M disturbance of inner-water circulation of biogenic substances is possible. Besides, at the increased content of ions of manganese perhaps formation in water highly-reactive oxidants - micro colloidal particles of mixed-valence manganese Mn(III,IV). Toxic super-oxidizing state of the water environment is formed.