A bubble column, subjected to low-frequency vibrations, displays maxima in the gas holdup when operated at certain frequencies. These maxima represent various harmonics created by standing waves. The axial distribution of gas holdup was measured for these harmonics to demonstrate that the gas holdup at the anti-nodes is higher than at the nodes; this phenomena is a manifestation of the primary Bjerknes force acting on the bubbles. The Bjerknes force can be exploited to obtain the optimum increase in the gas holdup for a given set of operating conditions. (c) 2005 Elsevier Ltd. All rights reserved.