In this study we analyzed the quality of local mixing in external loop air-lift bioreactors for two hydrodynamic regimes in terms of slip velocity. In particular. the effects of design and operating parameters (e.g., reactor geometry, superficial gas velocity and flow regime) on the slip velocity were determined. Several correlations found in the literature based on theoretical models of fluid flow and several semi-empirical and empirical correlations were examined and the applicability of all the proposed correlations was tested on available experimental data. The most accurate correlations for the prediction of the slip velocity in each bioreactor operating regime were identified. New correlations for homogeneous and heterogeneous flow regimes were developed. These correlations are among the most accurate and have increasing accuracy with increase of superficial gas velocity. They also give insight into how the change in geometry properties or gas flow will alter the slip velocity. This is very important in the design and optimization of air-lift bioreactors. (c) 2007 Elsevier B.V. All rights reserved.