The underground hypersaline brines of a calcium-chloride type of the Siberian platform differ considerably from sea water at a corresponding stage of evaporative concentration. The distinctions observed are explained by the additional gain of chemical elements from rocks with which the brines are not in equilibrium. The reaction state is determined by new thermodynamic calculations that take into account Pitzer's factors. The conclusion is that the water-rock system is an equilibrium-nonequilibrium one, ensuring the continuous geologically long-term disssolution of some minerals (aluminosilicates and sulphates) and formation of others (carbonates and clays). Because of this, some elements (Ca, Sr, Na etc.) are concentrated in the solution in conformity with physical-chemical laws.