The parallel competing iodide-iodate reaction scheme was used to investigate micromixing efficiencies in an aerated stirred tank with diameter T = 0.3 m. Half-elliptical blade-disc turbines (HEDT) with diameters (D) 76, 100, and 150 mm, corresponding to D/T of 0.25, 0.33, and 0.50, respectively, were used. The appropriate injection time and feed-pipe diameter were first determined to eliminate the effects of macromixing and backmixing on micromixing performance. The influences of impeller diameters D, power inputs P-m, superficial gas velocities V-s, and feed positions on the micromixing efficiency in the impeller discharging area were investigated. The results showed that HEDT with D/T 0.25 could be excluded from industrial applications because of the extremely high agitation speed N-cd for complete gas dispersion and poor micromixing performance in the presence of the gas. Given the same P-m, HEDT with smaller D/T showed better micromixing performance than that with bigger D/T, because the former leads to a higher pressure head H. As for the feed positions in the discharging area, the closer the feed position to the impeller, the better the micromixing performance. Generally, the introduced gas did not have a significant influence on micromixing in the discharging area of HEDT, except for a negative effect of aeration on micromixing when the tank was fed at the furthest feed position (0.3D away from the impeller tip along the radial direction).