The oxygen transfer capacity and the mixing time of an air-lift reactor have been investigated by using a cylindrical tank of 0. 5 m diameter with a vertical cylindrical draft tube and a single circular orifice. The oxygen transfer capacity is found to depend strongly on the orifice size and the sparging power per unit volume of liquid content and less on the geometric configuration of the draft tube. The mixing time, however, is appreciably influenced by the draft tube design. The optimum conditions for the present air-lift reactor design were obtained with respect to both the oxygen transfer and the mixing time characteristics.