Airlift reactors are pneumatically agitated reactors that have been widely used in industries, particularly in bioprocesses. Extensive studies about the flow dynamics in airlift column reactors exist; however, most of these studies have focused on global hydrodynamic parameters using conventional techniques. The local flow characteristics, such as the macro-mixing and the turbulence intensity, are crucial for reliable design and scale-up, and they remain unclear. This work focuses on studying the macro-mixing in a draft-tube airlift bioreactor utilizing an advanced flow dynamic measurement technique, computer automated radioactive particle tracking (CARPT). True residence time distribution analyses for the overall column as well as individual regions, i.e., the riser, the downcomer, the top, and the bottom regions, are conducted for the first time based on CARPT measured particle trajectories. The effects of the superficial gas velocity and the top/bottom clearances on the macro-mixing are also discussed. The results suggest that although the flow structures in the overall draft-tube column reactor, as well as in the riser and in the downcomer, are close to plug flows, bypassing and stagnancy exist in the top and the bottom regions. (C) 2007 Elsevier Ltd. All rights reserved.