Mixing time is determined at different probe and tracer injection positions, as shown in Table 3. A Rushton impeller located at T/3 above the bottom with a speed of 425 rpm is tested. The volume fraction of the dispersed phase is 15% and the gas flow rate is 0.40 L/ min. It is noted from Table 3 that the shortest mixing time is observed both at monitoring positions A and B when the tracer is injected into the liquid in the stirred tank from point P2. Moreover, the mixing time determined at A (upper probe position) is always longer than that at B (lower probe position). This phenomenon is consistent with the experimental observation (Distelhoff et al., 1997; Javed et al., 2006) and the numerical finding of Yeoh et al. (2005) from large eddy simulation. Yeoh et al. (2005) reported that the turbulent activity was much reduced in the region close to the top surface of the vessel. In order to attain the desired level of homogeneity at most parts of the whole vessel, the mixing time is determined by choosing the bigger value recorded among two probes in all later tests. The tracer is added at P1 in latter experiments for the sake of operation convenience. The aeration does not change the trend of influence of probe and injector positions on mixing time as compared with the experiment of Zhao et al. (2011).