Abstract: The electrochemical behaviours of the Al (Ⅲ)-Chromazol KS complex and the ligand have been investigated in HAc-NH Ac buffer solution (pH 6.1). The mechanism of related reactions is discussed. A new simple and sensitive single-sweep polarographic method is established for the determination of trace aluminium. The peak height of adsorptive complex wave is directly proportional to the concentration of Al(Ⅲ) in the range from 0.004 to 0.200 μg/ml, and the detection limit is 1.0ng/ml. The results of direct determination of trace aluminium in alloy steels are very satisfactory.