A novel competitive complexation/solvation theory has been developed for interpretation, simulation and preliminary quantification of extraction systems. This theory is based on the modified competitive preferential solvation theory, the concept of amphoteric properties of extractants and the concept of the formation of different aggregation structures on increasing extractant loading. The theory establishes correlation between complexation and solvation. Co-solvent independence allows direct comparison of the complexing/solvating power (competition order) of various solvents, used as extractants, enhancers (synergistic agents), and diluents. Once determined and tabulated, solute/solvent affinity constant ratio data may be used in any system containing these components. Mathematical descriptions of classical and the presented theories are compared and discussed. Some acid-amine extraction systems are analyzed based on the theory statements presented. The model provides a key for preliminary quantitative prediction of suitable extraction systems. Data, available from the literature and from experiments, were used. (C) 2003 Society of Chemical Industry.