Nitric thermal waters attract attention of scientists due to extraordinary composition: the water, infiltrating from the surface to the depth of 3-5 km of the Earth, heated up to 80-100 °С, and again appearing on the surface, is almost fresh. Its salinity is usually below 0,5 g/l, sometimes it is only 0,2-0,3 g/l. Together with this, in case of low salinity, the water has many elements in anomalously high concentrations, which are called redundant ones. Among these elements, there is Si, Na and F. Mechanisms of their accumulation remain unclear. The main aim of the study is to determine the main peculiarities of the nitric thermal waters composition in Transbaikalia region, their equilibrium with leading minerals of water-bearing rocks, and based on this information to study the accumulation mechanisms of redundant elements, including fluorine. The methods: thermodynamic calculations of thermal water equilibrium with main minerals, isotopic methods of its origin estimation and statistic methods of data analysis. The results. The paper introduces the data on chemical composition of the nitric thermal waters of Baikal Rift Zone. It is shown that the behavior of sulfate and carbonate ions in hydrothermal systems is different, which indicates that they are of two different origins. The studied thermal waters are of five chemical types formed in different geological conditions. Special attention is given to the thermal water equilibrium with basic minerals of water-bearing rocks, as well as their authigenic residuation. It was determined that thermal waters constantly dissolve endogenous aluminosilicates, accumulate chemical elements in solution, and form the secondary minerals of different composition after the equilibrium achievement. Together with this, a part of elements, including fluorine, is concentrated in the solution and becomes redundant elements. As calcium is connected by calcite, fluorine is concentrated in thermal waters quite long and, only after achieving the equilibrium with fluorite, precipitates from solution in the relevant form.