The gas-liquid mass transfer performance of different unbaffled dual-impeller mixers was investigated experimentally using low- and high-viscosity Newtonian and non-Newtonian liquids. The tested configurations were composed of two Rushton turbines located at two different height levels in the vessel in centered or off-centered positions. Various mixers were compared based on their respective mass transfer performance measured by means of two dissolved oxygen probes located at different levels in the vessel. k(l)a correlations and an axial homogeneity criterion were established for the different configurations and liquids used. It was concluded that the dual shaft mixer consisting of two off-centered shafts appears to be a very interesting and flexible configuration to deal with Newtonian liquids in a large range of viscosities. However, concerning the gas-liquid dispersion in non-Newtonian liquids, it is shown that off-centered shaft configurations have to be avoided. (C) 2007 Elsevier Ltd. All rights reserved.