Bioprocesses are operations especially sensitive to local concentration gradients of substrate or pH. These phenomena occur in large scale fermentations as a consequence of imperfect mixing. Therefore, prediction of mixing is of major importance for reactor design and process scale-up. A compartment model to describe mixing in a gas-liquid/multi-impeller system was developed and verified on a 30 m(3) large scale fermenter equipped with four Rushton turbines. A fluorescence pulse-response technique was employed to quantify mixing at different stirrer speeds and aeration rates in the regime of Limited recirculation. The influence of aeration on mixing was minor: at first negative (reduced mechanical power input), then positive (increased power transfer from gas). These trends were described qualitatively and quantitatively by the developed compartment model. Comparison between simulations and experiments shows that pulse-response curves and actual level of mixing time were predicted well. Stirrer speed, aeration rate, gas hold-up, power input and reactor geometry were utilized to obtain model parameter values. They are based on the general knowledge of hydrodynamics in this kind of vessels. That makes the model useful for (bio)process design and for further extension with microbial kinetics.