The equilibria and the kinetics of the A1 (III) complex formation with Semi-Xylenol Orange (SXO) in an aqueous solution have been investigated potentiometrically and spectrophotometrically. A1 (III) forms three complex species, A1SXO^-, A1HSXO and A1(OH)SXO^<2-> with SXO. The stability constants and the rate constants of these complex formations have been determined. The complex formation may proceed following the Eigen-Tamm mechanism, and the dissociation of the water molecule from the aquo ion may be the rate-determining of the complex formation.