Experiments were performed to investigate the mixed jet and mass transfer characteristics of a horizontal aeration process with the primary nozzle distance ratio of an annular nozzle ejector and the control valve opening. A direct visualization technique with a high speed camera was used to capture the images of the horizontal mixed jet, and a binarization technique was used to analyze the images. The clean water unsteady state technique was used for the oxygen mass transfer measurement. The measured data were used to calculate the air volume fraction, specific input power, volumetric mass transfer coefficient and mass transfer efficiency. Experimental results showed that the horizontally discharged mixed jet behaved like a buoyancy jet, a quasi-horizontal bubble jet, or a horizontal momentum jet due to the horizontal momentum of primary flow and the buoyancy force of the suction air bubble. It was found that the input power and air volume fraction decreased with the primary nozzle distance ratio, while the volumetric mass transfer coefficient and mass transfer efficiency increased and then decreased. It was also found that the average volumetric mass transfer coefficients with the primary nozzle distance ratio and Reynolds number for the full valve opening were higher than those for the 1/5 valve opening, while the average mass transfer efficiencies for the full valve opening were lower than those for the 1/5 valve opening. Based on the experimental results, an empirical correlation to predict the effects of Reynolds number, air volume fraction and specific input power on the volumetric mass transfer coefficient was proposed. (C) 2017 Elsevier Ltd. All rights reserved.