The quintessence of developed approach consists in the thermodynamic analysis of concurrent reactions in the homogeneous system Cu(I)-NH3–S2O3 2--H2O and heterogeneous systems Cu2O(S)-NH3–S2O3 2--H2O under real conditions on the basis of the introduced notion of the generalized reaction equation (GRE). The formation of mixed ligand complex species i j k Cu(OH) (NH ) (S O ) 3 2 3 is characterized by certain peculiarities in the behavior of studied two-phase heterogeneous systems. GRE allows complete chemical description of the overall process of mixed complex formation consisting of a series of concurrent reactions, where the nature and ratio of the concentrations of chemical species, formed in such reactions, depend on the ratio of the concentrations of the metal Cu2+ and ligands, temperature and other factors (thermodynamic parameters).