A combined computational fluid dynamics (CFD) and population balance model (PBM) approach has been applied to the simulation of gas-liquid stirred tanks agitated by (i) a Rushton turbine or (h) a CD-6 impeller, operating at aeration numbers from 0.017 to 0.038. The multiphase simulations were realised via an Eulerian-Eulerian two-fluid model and the drag coefficient of spherical and distorted bubbles was modelled using the Ishii-Zuber equations. The effect of the void fraction on the drag coefficient was modelled using the correlation by Behzadi et al. [Behzadi, A., Issa, R.I. and Rusche, H., 2004, Modelling of dispersed bubble and droplet flow at high phase fractions, Chem Eng Sci, 59: 759-770]. The local bubble size distribution was obtained by solving the PBM using the quadrature method of moments (QMOM). The local k(L)a was estimated using both the Higbie penetration theory and the surface renewal model. The predicted gas-liquid hydrodynamics, local bubble sizes and dissolved oxygen concentration were in good agreement with experimental measurements reported in the literature. A slight improvement in the prediction of the aerated power number was obtained using the non-uniform bubble size distribution resulting from the coupled CFD-PBM simulation. Evaluation of the prospective scale-up approaches indicates a higher probability of maintaining a similar level of mass transfer in a larger tank by keeping the P-g/V and VVM constant. Considering its predictive capability, the method outlined in this work can provide a useful scale-up evaluation of gas-liquid stirred tanks. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.