Long-term data from Rosgidromet and research results were used in comparing the chemistry condition of the southern reservoirs (Tsimlyansk, Proletarsk and Veselyi) to reveal that the ecologo-toxicological indices and characteristics of such intra-waterbody processes as pollution, eutrophication and toxification differ substantially. In the first place, the differences apply for the hydrochemical regime and mineralization as well as for individual biological indices. Common to the reservoirs are a stable (chronic) character of chemical contamination (in spite of its relatively low level), and a long-lasting presence of stable chlor¬organic pesticides in the composition of contamination, and a decline in biodiversity. The Tsimlyansk and Veselyi reservoirs show a similar seasonal dynamics of the phytoplankton, and a predominance of blue-green algae in the phytoplankton community. An unfavorable factor for the ecosystem of the Tsimlyansk reservoir is represented by annually recurring water bloom, with a large-scale occurrence of toxic species of algae, and by an increase in toxicity of natural water (toxification).Keywords: reservoirs in southern Russia, ecologo-toxicological state, mineralization, contaminants, water quality.