The chapter presents a thermodynamic approach for the complex chemical and redox equilibria investigation of mono- and two-phase systems containing metal ions of slightly soluble solid phases in different oxidation states. For heterogeneous systems, this approach utilizes thermodynamic relationships combined with original mass balance constraints. 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. On the basis of the introduced generalized equation of the overall redox reaction the physical meaning of the overall electrode potential under conditions other than standard ones, was revealed. The diagrams of complex chemical equilibria have been used for the graphical representation of complex equilibria in aqueous solutions. The developed approach can be applied to calculating potential-pH (Pourbaix) diagrams, based on the thermodynamic analysis of chemical equilibria in homogeneous and heterogeneous multicomponent systems.