The rate of gas induction was measured in gas-inducing-type mechanically agitated contactors (GIMAC) provided with two impellers. Three vessels of 0.57, 1.0, and 1.5 m i.d. were used. Tap water was used as the liquid phase, and air was used as the gas phase. Six different impeller designs (pitched-blade downflow and upflow turbines straight-blade turbine, disk turbine, and upflow and downflow propellers) were employed. From these designs, six different impeller combinations were made and an optimum combination has been proposed. The impeller speed was varied in the range of 0.30 to 15.45 rev/s. The ratio of impeller diameter to tank diameter (D/T) and the submergence (S) of upper impeller from the top were varied. The effects of clearance of lower impeller from the tank bottom (C-1) and the impeller spacing (C-3, distance between the two impellers) were also varied over a wide range. The design of the lower impeller was optimized in terms of diameter (D), blade width (W), blade angle (B-phi), number of blades (n(b)), and the blade thickness (t(b)). Rational correlations have been developed for the critical impeller speed for gas induction and the rate of gas induction.