The study of the hydrodynamics generated by impellers and its effect on the generation of bubbles and on their rising and dispersion is of key importance to improve the knowledge about the contact between phases and the mass transfer rates, particularly in cases where it is the limiting step. CFD simulations and high-speed video techniques are used to study the hydrodynamics developed by five different impellers, each located at three different positions above the dispersion device. Furthermore, two dispersion devices with one and two holes, respectively, are also used. The effect of the impellers on the characteristics of the bubbles and of the dispersions generated has been analysed. Bubbles generated under stirring are smaller than those generated in stagnant fluids. It is also shown that the initial bubble size at the orifice determines the contribution of the impeller and the perforated plate to the Sauter mean diameter. Although bubble formation is chaotic, the formation period is predictable based on three variables: the location of the impeller, its rotational speed and the gas flow rate. Bubble mean diameter was correlated to classical equations based on Kolmogorov's theory. Only when impellers are capable of breaking the bubbles, Kolmogorov's theory is completely verified. (C) 2008 Elsevier Ltd. All rights reserved.