The reversed-phase ion-pair liquid chromatographic behavior of the complex of aluminum ion with 5-sulfoquinoline-8-ol (HQS) was investigated on the column packed with a polystyrene-divinylbenzene copolymer with the mobile phase containing fluoride ion. It was found that the fluoride ion replaces one or two HQS molecules from the aluminum-HQS complex to form the mixed ligand complexes, which resulted in the appearance of two or more peaks in the chromatograms obtained for aluminum ion. The compositions of the mixed ligand complexes were estimated from the dependence of the areas of the peaks on the mobile phase concentrations of HQS and fluoride ion. The equilibrium and rate constants for the ligand exchange reaction were also evaluated and the reaction mechanism was presented. Aluminum ion could be completely separated from gallium ion by utilizing the ligand exchange reaction.