A thermodynamic approach for the complex chemical equilibria investigation of two-phase systems containing hydroxy aluminium sulfate (HAS) minerals in soils has been developed. This approach utilizes thermodynamic relationships combined with original mass balance constraints, where the HAS mineral phases are explicitly expressed. The factors influencing the distribution and concentrations of various soluble aluminium species have been taken into account. The new type of diagrams, based on thermodynamic, graphical and computerized methods, which quantitatively describe the distribution of soluble and insoluble inorganic, and organic, monomeric and polymeric aluminium species in acidic soil solutions in a large range of pH values has been used. The thermodynamics of equilibria of different natural HAS in soils, the conditions under which solids involving common ions can coexist at equilibrium, the acid-base and mineral equilibria and complex formation have been examined. It has been proved that the presence of sulfate ion dramatically alters the HAS mineral solubility under acidic conditions. The obtained data regarding the factors influencing Al speciation, based on the constructed diagrams of heterogeneous chemical equilibria, are in good agreement with the current experimental and theoretical evidence. The proposed approach is intended to determine the dominant processes that are responsible for the Al³⁺ concentration levels and its speciation in acidic soils. [Figure not available: see fulltext.]