In this paper we show that application of low-frequency vibrations, in the 40-120 Hz range, to the liquid phase of an air-water bubble column causes significantly smaller bubbles to be generated at the nozzle. In experiments with a single capillary nozzle, the bubble size is reduced by about 40-50%, depending on the vibration frequency and amplitude. CFD simulations show that the vibrations tend to lead to earlier detachment of the bubbles from the nozzles, leading to smaller bubble sizes. Using a 12-capillary nozzle arrangement, the gas holdup, epsilon, was measured for a range of superficial gas velocities. Application of vibrations to the liquid phase leads to a significant increase in the gas holdup. The increase in the gas holdup is attributed mainly to a significant reduction in the rise velocity of the bubble swarm due to the generation of standing waves in the column. Furthermore, application of vibrations to the liquid phase serves to stabilize the homogenous bubbly flow regime and delay the onset of the churn-turbulent flow regime. (C) 2003 Elsevier Science B.V. All rights reserved.