Simultaneous measurements of liquid velocity by laser Doppler velocimetry and bubble velocity, diameter, and void fraction by a double optical probe are performed in a bubble column to study the influence of the void fraction on the relative velocity of a swarm of gas bubbles. Bubble diameters d(b) vary from 2 to 10mm and local void fractions alpha(loc) can reach 35%. It is found that, for aloe < 15%, the relative bubble velocity is determined by the hindrance effect and consequently decreases with the void fraction. Beyond this critical value, the aspiration of bubbles in the wake of the leading ones dominates the hindrance effect and the relative velocity thus increases suddenly. The contribution of the bubble diameters to this evolution is also determined. Finally, a drag correlation,. valid for the whole range of void fraction and for pure water-air systems, is proposed. (c) 2006 Elsevier Ltd. All rights reserved.