The authors show a new thermodynamic approach for studying chemical equilibria under real conditions, taking into account the complex formation reactions in multicomponent heterogeneous systems. Its quintessence consists in the thermodynamic analysis of the conditions of various processes on the basis of their total thermodynamic characteristics. In the case of taking place of reactions of complex formation in heterogeneous multicomponent systems, the total change of Gibbs energy cannot be presented as a sum of contributions of several separate reactions. An original method for the determination of enthalpies of reactions on the basis of temperature coefficients of the indexes of the metal ion equilibrium concentration, determined experimentally, has been developed. The authors have been introduced a number of new thermodynamic characteristics for the complex formation and precipitation-dissolving reactions as the degree of formation of polynuclear complexes, the stability coefficient of system components, the degree of transformation of one phase to another etc. Novel methods for determining such quantities as the solubility and solubility product, along with the stability constant of chemical compounds of the arbitrary composition in the heterogeneous multicomponent systems are also described.