Several impellers, perforated plates and geometrical configurations were tested in order to evaluate the effect of the particular hydrodynamics generated by each impeller on the mass transfer rates and to optimize the performance of the tank. Theoretical and empirical equations have been used or proposed, based on the experimental data, to study the oxygen transfer rates from air bubbles generated in a non-standard stirred tank. The empirical equations obtained depend on the impeller type, its position and the design of the perforated plate because of their effect on the bubbles. The optimal position of the impeller depends on the physical effect of the impeller on the bubbles. Higher mass transfer coefficients were obtained close to the perforated plates. Not only the dispersion but also the break up of the bubbles favors the mass transfer rates. In short, although the Rushton turbine is efficient and stable with its relative position, other impellers show very interesting results for lower power inputs. (C) 2008 Elsevier Ltd. All rights reserved.