Air entrainment rate and holdup are important and potential control parameters in plunging liquid jet bubble columns. In this study, air entrainment and holdup in laboratory Jameson cell downcomer were investigated in an air-water system. Air entrainment flow rates and holdup were quantified experimentally. The effects of various conditions in the nozzle diameter, the downcomer diameter, the free jet length, the jet velocity and the ratio of air-feed flow rate on gas holdup and air entrainment were studied. Two empirical equations were proposed to estimate gas holdup. The predictions from these equations are in good agreement with the experimental data for holdup. (c) 2007 Elsevier Ltd. All rights reserved.