Competitive complexation/solvation theory is used to interpret the influence of active solvents on solvent extraction of acids by amine-based extractants. This theory is based on the modified competitive preferential solvation theory, the concept of amphoteric properties of extractants, and the concept of changing aggregation structures when extractant loading increases. Active solvent concentration parameter is introduced to the quantitative consideration of the extraction systems. The paper discusses four possible stages of extraction behavior, interaction mechanisms, and aggregate structures formation, depending on solute-solvent affinity constant and concentration ratios. The influence of different types of active solvents in each stage is analyzed. Mathematical description is introduced for the quantification of results and process simulation. It gives a key for preliminary quantitative prediction of suitable extraction systems.