The liquid-phase mixing in gassed vessels agitated by up- and downward-pumping 30 degrees and 45 degrees pitched-blade turbines has been investigated using the radioactive tracer technique called Positron Emission Particle Tracking (PEPT). Only the flow patterns of the 45 degrees down-pumping pitched-blade turbine were significantly affected by the injection of gas at specific flow rates of up to 1.5 vvm. A fluid agitation index was used to quantify the effectiveness of each impeller, based on the ratio of the average fluid velocity and the impeller tip velocity. At a given agitation speed, the 45 degrees pitched-blade turbines were the most efficient at transferring momentum to the fluid, as indicated by higher circulation indexes. The efficiency of the 45 degrees down-pumping pitched-blade turbine was significantly reduced upon aeration of the vessel, even at low specific gas flow rates of 0.5 vvm.