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The flow scheme and oxygen transfer were quantitatively investigated for the gas-liquid downflow system. Experiments were carried out on a pilot-scale column, 45 cm in inner diameter and 6 m high. The slip velocity of gas bubble swarm against liquid phase was correlated with gas holdup and bubble diameter, and thus conditions giving stable downward flow of completely segregated gas bubbles were specified. The fraction of oxygen absorbed into liquid phase was observed under stable bubble flow conditions. Much higher volumetric oxygen transfer coefficients in the present system than those in the conventional upflow system were obtained. This was attributed to the high values of gas holdup, even up to 20%, in the present system. It is suggested that the present system could be applied as a promising aeration device utilizing enriched oxygen gas.