The paper presents a new thermodynamic approach to studying mixed ligand complex formation reactions in multicomponent two-phase systems “solid phase saturated aqueous solution” under real conditions. The quintessence of developed approach consists in the thermodynamic analysis of concurrent reactions in the system Cu (II)-NH3-S2O32--H2O under real conditions on the basis of the introduced notion of the generalized reaction equation. The formation of mixed ligand complex species Cu(OH)i (NH3 ) j (S2O3 )k is characterized by certain peculiarities in the behavior of studied two-phase heterogeneous systems. The new feature is to focus on the chemical description of the overall process of 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).