Thermodynamic approach for the complex chemical and redox equilibria investigation of mono- and two-phase systems containing metal ion hydroxides in different oxidation states was developed. This approach utilizes thermodynamic relationships combined with original mass balance constraints, where the solid phases are explicitly expressed. The forms of occurrence of metal ions are determined by the major chemical reactions in the aquatic environment such as hydrolysis, oxidation, reduction, and precipitation. Depending on pH, potential and total concentration of inorganic and organic ligands, metal ions may undergo various transformations to produce a whole range of chemical species in aqueous systems. The factors influencing the distribution and amounts of various soluble metal ion species have been taken into account. The new type of ΔG–pH diagrams, based on thermodynamic, graphical and computerized methods, which quantitatively describe the distribution of soluble and insoluble, monomeric and polymeric metal ion species in a large range of pH values has been used. By means of the ΔG–pH diagrams, the areas of thermodynamic stability of metal ion hydroxides in different oxidation states have been established for a number of analytical concentrations in heterogeneous mixtures. The diagrams of heterogeneous and homogeneous chemical equilibria have been used for graphical representation of complex equilibria in aqueous solutions. The developed approach has been applied for calculating potential−pH (Pourbaix) diagrams, based on the thermodynamic analysis of chemical equilibria in the homogeneous and heterogeneous multicomponent systems.